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95-531: The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way . It was originally named the Andromeda Nebula and is cataloged as Messier 31 , M31 , and NGC 224 . Andromeda has a D 25 isophotal diameter of about 46.56 kiloparsecs (152,000 light-years ) and is approximately 765 kpc (2.5 million light-years) from Earth. The galaxy's name stems from

190-462: A main sequence , subgiant , or red giant star . If the orbital period of the system is a few days or less, the white dwarf is close enough to its companion star to draw accreted matter onto its surface, creating a dense but shallow atmosphere . This atmosphere, mostly consisting of hydrogen, is heated by the hot white dwarf and eventually reaches a critical temperature, causing ignition of rapid runaway fusion . The sudden increase in energy expels

285-501: A solar mass , quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused during the power outburst. Nonetheless, this is enough energy to accelerate nova ejecta to velocities as high as several thousand kilometers per second—higher for fast novae than slow ones—with a concurrent rise in luminosity from a few times solar to 50,000–100,000 times solar. In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that

380-462: A bar. The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting farther out, which creates a self-perpetuating bar structure. The bar structure is believed to act as a type of stellar nursery , channeling gas inwards from the spiral arms through orbital resonance , fueling star birth in

475-429: A binary system. One of the two evolves into a red giant , leaving its remnant white dwarf core in orbit with the remaining star. The second star—which may be either a main-sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche lobe . As a result, the white dwarf steadily captures matter from the companion's outer atmosphere in an accretion disk, and in turn,

570-677: A classical nova may brighten by more than 12 magnitudes. Although it is estimated that as many as a quarter of nova systems experience multiple eruptions, only ten recurrent novae (listed below) have been observed in the Milky Way. Several extragalactic recurrent novae have been observed in the Andromeda Galaxy (M31) and the Large Magellanic Cloud . One of these extragalactic novae, M31N 2008-12a , erupts as frequently as once every 12 months. On 20 April 2016,

665-405: A distance estimate of 500,000 ly (3.2 × 10 AU). Although this estimate is about fivefold lower than the best estimates now available, it was the first known estimate of the distance to Andromeda that was correct to within an order of magnitude (i.e., to within a factor of ten of the current estimates, which place the distance around 2.5 million light-years). Curtis became a proponent of

760-410: A drawing of Andromeda's spiral structure . In 1864, William Huggins noted that the spectrum of Andromeda differed from that of a gaseous nebula. The spectrum of Andromeda displays a continuum of frequencies , superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is very similar to the spectra of individual stars, and from this, it

855-516: A galaxy in the B-band (445 nm wavelength of light, in the blue part of the visible spectrum ) reaches 25 mag/arcsec. The Third Reference Catalogue of Bright Galaxies (RC3) used this standard for Andromeda in 1991, yielding an isophotal diameter of 46.56 kiloparsecs (152,000 light-years) at a distance of 2.5 million light-years. An earlier estimate from 1981 gave a diameter for Andromeda at 54 kiloparsecs (176,000 light-years). A study in 2005 by

950-525: A galaxy that lies in the "green valley" of the Galaxy color-magnitude diagram (see below ). Supernovae erupt in the Andromeda Galaxy's star-filled disk and eject these heavier elements into space. Over the Andromeda Galaxy's lifetime, nearly half of the heavy elements made by its stars have been ejected far beyond the galaxy's 200,000-light-year-diameter stellar disk. The estimated luminosity of

1045-553: A giant elliptical galaxy or a large lenticular galaxy . With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects , and is visible to the naked eye from Earth on moonless nights, even when viewed from areas with moderate light pollution . The Andromeda Galaxy is visible to the naked eye in dark skies. Around the year 964 CE , the Persian astronomer Abd al-Rahman al-Sufi described

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1140-422: A great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation. Should this continue, the luminosity of the Milky Way may eventually overtake that of the Andromeda Galaxy. According to recent studies, the Andromeda Galaxy lies in what is known in the galaxy color–magnitude diagram as the "green valley", a region populated by galaxies like

1235-453: A lesser one at −7.5. Novae also have roughly the same absolute magnitude 15 days after their peak (−5.5). Nova-based distance estimates to various nearby galaxies and galaxy clusters have been shown to be of comparable accuracy to those measured with Cepheid variable stars . A recurrent nova ( RN ) is an object that has been seen to experience repeated nova eruptions. The recurrent nova typically brightens by about 9 magnitudes, whereas

1330-449: A metallicity correction of −0.2 mag dex in (O/H), an estimate of 2.57 ± 0.06 million light-years (1.625 × 10 ± 3.8 × 10 astronomical units ) was derived. A 2004 Cepheid variable method estimated the distance to be 2.51 ± 0.13 million light-years (770 ± 40 kpc). In 2005, an eclipsing binary star was discovered in the Andromeda Galaxy. The binary is made up of two hot blue stars of types O and B. By studying

1425-421: A nova also can emit gamma rays (>100 MeV). Potentially, a white dwarf can generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion star. Where this repeated flaring is observed, the object is called a recurrent nova. An example is RS Ophiuchi , which is known to have flared seven times (in 1898, 1933, 1958, 1967, 1985, 2006, and 2021). Eventually,

1520-480: A nova is less dependent on the accretion rate of the white dwarf than on its mass; with their powerful gravity, massive white dwarfs require less accretion to fuel an eruption than lower-mass ones. Consequently, the interval is shorter for high-mass white dwarfs. V Sagittae is unusual in that the time of its next eruption can be predicted fairly accurately; it is expected to recur in approximately 2083, plus or minus about 11 years. Novae are classified according to

1615-508: A radius of 33,000 ly (2.1 × 10 AU), where it reaches a peak of 250 km/s (160 mi/s). The velocities slowly decline beyond that distance, dropping to around 200 km/s (120 mi/s) at 80,000 ly (5.1 × 10 AU). These velocity measurements imply a concentrated mass of about 6 × 10  M ☉ in the nucleus . The total mass of the galaxy increases linearly out to 45,000 ly (2.8 × 10 AU), then more slowly beyond that radius. The spiral arms of

1710-426: A sign of galaxies reaching full maturity as the "formative years" end. A 2008 investigation found that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with about 65 percent of their local counterparts. The general classification is "SB" (spiral barred). The sub-categories are based on how open or tight the arms of the spiral are. SBa types feature tightly bound arms. SBc types are at

1805-575: A single source named 3XMM J004232.1+411314 , and identified as a binary system where a compact object (a neutron star or a black hole) accretes matter from a star. Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from the European Space Agency 's (ESA) XMM-Newton orbiting observatory. Robin Barnard et al. hypothesized that these are candidate black holes or neutron stars , which are heating

1900-426: Is a black hole at its center. Apparently, by late 1968, no X-rays had been detected from the Andromeda Galaxy. A balloon flight on 20 October 1970 set an upper limit for detectable hard X-rays from the Andromeda Galaxy. The Swift BAT all-sky survey successfully detected hard X-rays coming from a region centered 6 arcseconds away from the galaxy center. The emission above 25 keV was later found to be originating from

1995-401: Is actually a barred spiral galaxy , like the Milky Way, with Andromeda's bar major axis oriented 55 degrees anti-clockwise from the disc major axis. There are various methods used in astronomy in defining the size of a galaxy, and each method can yield different results concerning one another. The most commonly employed is the D 25 standard, the isophote where the photometric brightness of

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2090-534: Is heavily reddened by the Andromeda Galaxy's interstellar dust , was thought to be more massive than G1 and the largest cluster of the Local Group; however, other studies have shown it is actually similar in properties to G1. Barred spiral galaxy Edwin Hubble classified spiral galaxies of this type as "SB" (spiral, barred) in his Hubble sequence and arranged them into sub-categories based on how open

2185-493: Is hidden from visible light images of the galaxy because it is composed primarily of cold dust, and most of the star formation that is taking place in the Andromeda Galaxy is concentrated there. Later studies with the help of the Spitzer Space Telescope showed how the Andromeda Galaxy's spiral structure in the infrared appears to be composed of two spiral arms that emerge from a central bar and continue beyond

2280-412: Is measured to be ≈ 160  km/s (100  mi/s ). It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole. The eccentricity is such that stars linger at the orbital apocenter , creating a concentration of stars. It has been postulated that such an eccentric disk could have been formed from

2375-426: Is only half of the Milky Way's newer mass, calculated in 2019 at 1.5 × 10  M ☉ . In addition to stars, the Andromeda Galaxy's interstellar medium contains at least 7.2 × 10  M ☉ in the form of neutral hydrogen , at least 3.4 × 10  M ☉ as molecular hydrogen (within its innermost 10 kiloparsecs), and 5.4 × 10  M ☉ of dust . The Andromeda Galaxy

2470-458: Is surrounded by a massive halo of hot gas that is estimated to contain half the mass of the stars in the galaxy. The nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our Milky Way Galaxy. Simulations of galaxies indicate the halo formed at the same time as the Andromeda Galaxy. The halo is enriched in elements heavier than hydrogen and helium, formed from supernovae , and its properties are those expected for

2565-483: Is the second-brightest galaxy within a radius of 10 megaparsecs of the Milky Way, after the Sombrero Galaxy , with an absolute magnitude of around −22.21 or close). An estimation done with the help of Spitzer Space Telescope published in 2010 suggests an absolute magnitude (in the blue) of −20.89 (that with a color index of +0.63 translates to an absolute visual magnitude of −21.52, compared to −20.9 for

2660-523: The Sky & Telescope website reported a sustained brightening of T Coronae Borealis from magnitude 10.5 to about 9.2 starting in February 2015. A similar event had been reported in 1938, followed by another outburst in 1946. By June 2018, the star had dimmed slightly but still remained at an unusually high level of activity. In March or April 2023, it dimmed to magnitude 12.3. A similar dimming occurred in

2755-656: The 2C radio astronomy catalog. In 2009, an occurrence of microlensing —a phenomenon caused by the deflection of light by a massive object—may have led to the first discovery of a planet in the Andromeda Galaxy. In 2020, observations of linearly polarized radio emission with the Westerbork Synthesis Radio Telescope , the Effelsberg 100-m Radio Telescope , and the Very Large Array revealed ordered magnetic fields aligned along

2850-472: The Cartwheel encounter . Studies of the extended halo of the Andromeda Galaxy show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally " metal-poor ", and increasingly so with greater distance. This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 100–200 low-mass galaxies during

2945-410: The European Space Agency 's Infrared Space Observatory demonstrated that the overall form of the Andromeda Galaxy may be transitioning into a ring galaxy . The gas and dust within the galaxy are generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 ly (9.8 kpc) from the core, nicknamed by some astronomers the ring of fire . This ring

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3040-493: The Hubble Space Telescope was used to image the Andromeda Galaxy's inner nucleus. The nucleus consists of two concentrations separated by 1.5  pc (4.9  ly ). The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains an embedded star cluster, called P3, containing many UV -bright A-stars and

3135-559: The Keck telescopes shows the existence of a tenuous sprinkle of stars, or galactic halo , extending outward from the galaxy. The stars in this halo behave differently from the ones in Andromeda's main galactic disc, where they show rather disorganized orbital motions as opposed to the stars in the main disc having more orderly orbits and uniform velocities of 200 km/s. This diffuse halo extends outwards away from Andromeda's main disc with

3230-550: The Triangulum Galaxy (M33) might have had a very close passage 2–4 billion years ago, but it seems unlikely from the last measurements from the Hubble Space Telescope. At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy. In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value and

3325-459: The interstellar medium is not great; novae supply only 1 ⁄ 50 as much material to the galaxy as do supernovae, and only 1 ⁄ 200 as much as red giant and supergiant stars. Observed recurrent novae such as RS Ophiuchi (those with periods on the order of decades) are rare. Astronomers theorize, however, that most, if not all, novae recur, albeit on time scales ranging from 1,000 to 100,000 years. The recurrence interval for

3420-412: The light curve decay speed, referred to as either type A, B, C and R, or using the prefix "N": Some novae leave behind visible nebulosity , material expelled in the nova explosion or in multiple explosions. Novae have some promise for use as standard candle measurements of distances. For instance, the distribution of their absolute magnitude is bimodal , with a main peak at magnitude −8.8, and

3515-401: The rotational velocity of the Andromeda Galaxy as a function of radial distance from the core. The rotational velocity has a maximum value of 225 km/s (140 mi/s) at 1,300  ly (82,000,000  AU ) from the core, and it has its minimum possibly as low as 50 km/s (31 mi/s) at 7,000 ly (440,000,000 AU) from the core. Further out, rotational velocity rises out to

3610-409: The supermassive black hole , called M31* . The black hole is classified as a low-luminosity AGN (LLAGN) and it was detected only in radio wavelengths and in x-rays . It was quiescent in 2004–2005, but it was highly variable in 2006–2007. The mass of M31* was measured at 3–5 × 10 M ☉ in 1993, and at 1.1–2.3 × 10 M ☉ in 2005. The velocity dispersion of material around it

3705-562: The "10-kpc ring" of gas and star formation. The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is a second, dimmer type of Cepheid variable star . In the 1990s, measurements of both standard red giants as well as red clump stars from the Hipparcos satellite measurements were used to calibrate the Cepheid distances. A major merger occurred 2 to 3 billion years ago at

3800-416: The Andromeda Galaxy are outlined by a series of HII regions , first studied in great detail by Walter Baade and described by him as resembling "beads on a string". His studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy. His descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows: Since

3895-583: The Andromeda Galaxy in his Book of Fixed Stars as a "nebulous smear" or "small cloud". Star charts of that period labeled it as the Little Cloud . In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations. Pierre Louis Maupertuis conjectured in 1745 that the blurry spot was an island universe. Charles Messier cataloged Andromeda as object M31 in 1764 and incorrectly credited Marius as

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3990-407: The Andromeda Galaxy is seen close to edge-on, it is difficult to study its spiral structure. Rectified images of the galaxy seem to show a fairly normal spiral galaxy, exhibiting two continuous trailing arms that are separated from each other by a minimum of about 13,000  ly (820,000,000  AU ) and that can be followed outward from a distance of roughly 1,600 ly (100,000,000 AU) from

4085-421: The Andromeda Galaxy's spheroid was determined to have a higher stellar density than that of the Milky Way, and its galactic stellar disk was estimated at twice the diameter of that of the Milky Way. The total mass of the Andromeda Galaxy is estimated to be between 8 × 10  M ☉ and 1.1 × 10  M ☉ . The stellar mass of M31 is 10–15 × 10  M ☉ , with 30% of that mass in

4180-462: The Andromeda Galaxy's halo (including dark matter ) gave a value of approximately 1.5 × 10  M ☉ , compared to 8 × 10  M ☉ for the Milky Way. This contradicted even earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass. In 2018, the earlier measurements for equality of mass were re-established by radio results as approximately 8 × 10  M ☉ . In 2006,

4275-403: The Andromeda Galaxy, ~2.6 × 10  L ☉ , is about 25% higher than that of our own galaxy. However, the galaxy has a high inclination as seen from Earth, and its interstellar dust absorbs an unknown amount of light, so it is difficult to estimate its actual brightness and other authors have given other values for the luminosity of the Andromeda Galaxy (some authors even propose it

4370-420: The Andromeda location, involving two galaxies with a mass ratio of approximately 4. The discovery of a recent merger in the Andromeda galaxy was first based on interpreting its anomalous age-velocity dispersion relation, as well as the fact that 2 billion years ago, star formation throughout Andromeda's disk was much more active than today. Modeling of this violent collision shows that it has formed most of

4465-487: The German philosopher Immanuel Kant proposed the hypothesis that the Milky Way is only one of many galaxies in his book Universal Natural History and Theory of the Heavens . Arguing that a structure like the Milky Way would look like a circular nebula viewed from above and like an ellipsoid if viewed from an angle, he concluded that the observed elliptical nebulae like Andromeda, which could not be explained otherwise at

4560-545: The Milky Way Galaxy. There are approximately 460 globular clusters associated with the Andromeda Galaxy. The most massive of these clusters, identified as Mayall II , nicknamed Globular One, has a greater luminosity than any other known globular cluster in the Local Group of galaxies. It contains several million stars and is about twice as luminous as Omega Centauri , the brightest known globular cluster in

4655-435: The Milky Way in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties to the Andromeda Galaxy, star formation is expected to extinguish within about five billion years, even accounting for

4750-463: The Milky Way), and a total luminosity in that wavelength of 3.64 × 10  L ☉ . The rate of star formation in the Milky Way is much higher, with the Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of novae in the Milky Way is also double that of the Andromeda Galaxy. This suggests that the latter once experienced

4845-446: The Milky Way. In the Andromeda Galaxy , roughly 25 novae brighter than about 20th magnitude are discovered each year, and smaller numbers are seen in other nearby galaxies. Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. Classical nova explosions are galactic producers of the element lithium . The contribution of novae to

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4940-433: The Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by Andromeda in the distant past. The globular with the greatest apparent brightness is G76 which is located in the southwest arm's eastern half. Another massive globular cluster, named 037-B327 and discovered in 2006 as

5035-462: The accreted matter falls into the atmosphere. As the white dwarf consists of degenerate matter , the accreted hydrogen is unable to expand even though its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K , initiating nuclear burning via the CNO cycle . If the accretion rate is just right, hydrogen fusion may occur in a stable manner on

5130-465: The area of Earth's sky in which it appears, the constellation of Andromeda , which itself is named after the princess who was the wife of Perseus in Greek mythology . The virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at 1  trillion solar masses (2.0 × 10 kilograms ). The mass of either galaxy is difficult to estimate with any accuracy, but it

5225-568: The arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between the two. SB0 is a barred lenticular galaxy . A new type, SBm, was subsequently created to describe somewhat irregular barred spirals , such as the Magellanic Clouds , which were once classified as irregular galaxies, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for

5320-428: The atmosphere into interstellar space, creating the envelope seen as visible light during the nova event. In past centuries such an event was thought to be a new star. A few novae produce short-lived nova remnants , lasting for perhaps several centuries. A recurrent nova involves the same processes as a classical nova, except that the nova event repeats in cycles of a few decades or less as the companion star again feeds

5415-541: The bar compromises the stability of the overall bar structure. Simulations show that many bars likely experience a "buckling" event in which a disturbance in the orbital resonances of stars in the bar structure leads to an inward collapse in which the bar becomes thicker and shorter though the exact mechanism behind this buckling instability remains hotly debated. Barred spiral galaxies with high mass accumulated in their center thus tend to have short, stubby bars. Such buckling phenomena are significantly suppressed and delayed by

5510-512: The central bulge , 56% in the disk , and the remaining 14% in the stellar halo . The radio results (similar mass to the Milky Way Galaxy) should be taken as likeliest as of 2018, although clearly, this matter is still under active investigation by several research groups worldwide. As of 2019, current calculations based on escape velocity and dynamical mass measurements put the Andromeda Galaxy at 0.8 × 10  M ☉ , which

5605-401: The core. Alternative spiral structures have been proposed such as a single spiral arm or a flocculent pattern of long, filamentary, and thick spiral arms. The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites M32 and M110 . This can be seen by the displacement of the neutral hydrogen clouds from the stars. In 1998, images from

5700-414: The dense atmosphere of the white dwarf after each ignition, as in the star T Coronae Borealis . Under certain conditions, mass accretion can eventually trigger runaway fusion that destroys the white dwarf rather than merely expelling its atmosphere. In this case, the event is usually classified as a Type Ia supernova . Novae most often occur in the sky along the path of the Milky Way , especially near

5795-616: The diameter of 67.45 kiloparsecs (220,000 light-years). The galaxy is inclined an estimated 77° relative to Earth (where an angle of 90° would be edge-on). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk. A possible cause of such a warp could be gravitational interaction with the satellite galaxies near the Andromeda Galaxy. The Galaxy M33 could be responsible for some warp in Andromeda's arms, though more precise distances and radial velocities are required. Spectroscopic studies have provided detailed measurements of

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5890-469: The discoverer despite it being visible to the naked eye. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae ", and based on the color and magnitude of the nebula , he incorrectly guessed that it was no more than 2,000 times the distance of Sirius , or roughly 18,000  ly (5.5  kpc ). In 1850, William Parsons, 3rd Earl of Rosse , made

5985-578: The distance estimate to Andromeda, as well as the remainder of the universe. In 1950, radio emissions from the Andromeda Galaxy were detected by Robert Hanbury Brown and Cyril Hazard at the Jodrell Bank Observatory . The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group . The core of the Andromeda Galaxy is called 2C 56 in

6080-403: The dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. The main sub-classes of novae are classical novae, recurrent novae (RNe), and dwarf novae . They are all considered to be cataclysmic variable stars . Classical nova eruptions are the most common type. This type is usually created in a close binary star system consisting of a white dwarf and either

6175-534: The dust clouds in our own galaxy, as well as historical observations of the Andromeda Galaxy's significant Doppler shift . In 1922, Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450 kpc (1,500 kly). Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for

6270-403: The eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their absolute magnitude . When the visual and absolute magnitudes are known, the distance to the star can be calculated. The stars lie at a distance of 2.52 × 10 ^  ± 0.14 × 10 ^  ly (1.594 × 10 ± 8.9 × 10 AU) and

6365-417: The expected, short-term increase in the rate of star formation due to the collision between the Andromeda Galaxy and the Milky Way. Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs–Sandage extended classification system of spiral galaxies. However, infrared data from the 2MASS survey and the Spitzer Space Telescope showed that Andromeda

6460-466: The first photographs of Andromeda, which was still commonly thought to be a nebula within our galaxy. Roberts mistook Andromeda and similar "spiral nebulae" as star systems being formed . In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to the Solar System —the largest velocity yet measured, at 300 km/s (190 mi/s). As early as 1755,

6555-498: The first time on astronomical photos of Andromeda. These were made using the 100-inch (2.5 m) Hooker telescope , and they enabled the distance of the Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an entirely separate galaxy located a significant distance from the Milky Way. In 1943, Walter Baade

6650-588: The fixed stars, and thus the nova had to be very far away. Although SN 1572 was later found to be a supernova and not a nova, the terms were considered interchangeable until the 1930s. After this, novae were called classical novae to distinguish them from supernovae, as their causes and energies were thought to be different, based solely on the observational evidence. Although the term "stella nova" means "new star", novae most often take place on white dwarfs , which are remnants of extremely old stars. Evolution of potential novae begins with two main sequence stars in

6745-483: The galactic center and has about 10 M ☉ . It was discovered through data collected by the European Space Agency 's XMM-Newton probe and was subsequently observed by NASA 's Swift Gamma-Ray Burst Mission and Chandra X-Ray Observatory , the Very Large Array , and the Very Long Baseline Array . The microquasar was the first observed within the Andromeda Galaxy and the first outside of

6840-473: The galaxies are the spiral galaxy, elliptical galaxy and irregular galaxy. Although theoretical models of galaxy formation and evolution had not previously expected galaxies becoming stable enough to host bars very early in the universe's history, evidence has recently emerged of the existence of numerous spiral galaxies in the early universe. Barred galaxies are apparently predominant, with surveys showing that up to two-thirds of all spiral galaxies develop

6935-491: The galaxy's (metal-rich) galactic halo , including the Giant Stream, and also the extended thick disk, the young age thin disk, and the static 10 kpc ring. During this epoch, its rate of star formation would have been very high , to the point of becoming a luminous infrared galaxy for roughly 100 million years. Modeling also recovers the bulge profile, the large bar, and the overall halo density profile. Andromeda and

7030-403: The incoming gas to millions of kelvins and emitting X-rays. Neutron stars and black holes can be distinguished mainly by measuring their masses. An observation campaign of NuSTAR space mission identified 40 objects of this kind in the galaxy. In 2012, a microquasar , a radio burst emanating from a smaller black hole was detected in the Andromeda Galaxy. The progenitor black hole is located near

7125-416: The large ring mentioned above. Those arms, however, are not continuous and have a segmented structure. Close examination of the inner region of the Andromeda Galaxy with the same telescope also showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through the disk of the Andromeda Galaxy along

7220-409: The latter's polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda. It is the co-existence of the long-known large ring-like feature in the gas of Messier 31, together with this newly discovered inner ring-like structure, offset from the barycenter , that suggested a nearly head-on collision with the satellite M32, a milder version of

7315-460: The observed Galactic Center in Sagittarius; however, they can appear anywhere in the sky. They occur far more frequently than galactic supernovae , averaging about ten per year in the Milky Way. Most are found telescopically, perhaps only one every 12–18 months reaching naked-eye visibility. Novae reaching first or second magnitude occur only a few times per century. The last bright nova

7410-520: The other extreme and have loosely bound arms. SBb galaxies lie in between. SBm describes somewhat irregular barred spirals. SB0 is a barred lenticular galaxy . of barred Magellanic spiral Novae A nova ( pl. novae or novas ) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", Latin for "new") that slowly fades over weeks or months. All observed novae involve white dwarfs in close binary systems , but causes of

7505-412: The past 12 billion years. The stars in the extended halos of the Andromeda Galaxy and the Milky Way may extend nearly one-third the distance separating the two galaxies. The Andromeda Galaxy is known to harbor a dense and compact star cluster at its very center, similar to our own galaxy . A large telescope creates a visual impression of a star embedded in the more diffuse surrounding bulge. In 1991,

7600-432: The peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used for grouping novae into speed classes. Fast novae typically will take less than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days. Despite its violence, usually the amount of material ejected in a nova is only about 1 ⁄ 10,000 of

7695-409: The presence of a supermassive black hole in the galactic center but occur nonetheless. Since so many spiral galaxies have bar structures, it is likely that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on average about two billion years. Recent studies have confirmed the idea that bars are

7790-424: The result of a previous black hole merger, where the release of gravitational waves could have "kicked" the stars into their current eccentric distribution. P2 also contains a compact disk of hot, spectral-class A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1. While at the initial time of its discovery it

7885-498: The so-called "island universes" hypothesis: that spiral nebulae were actually independent galaxies. In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe . To support his claim that the Great Andromeda Nebula is, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda that resembled

7980-506: The surface of the white dwarf, giving rise to a supersoft X-ray source , but for most binary system parameters, the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other, heavier chemical elements in a runaway reaction, liberating an enormous amount of energy. This blows the remaining gases away from the surface of the white dwarf and produces an extremely bright outburst of light. The rise to peak brightness may be very rapid, or gradual; after

8075-407: The time, were indeed galaxies similar to the Milky Way, not nebulae, as Andromeda was commonly believed to be. In 1917, Heber Curtis observed a nova within Andromeda. After searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with

8170-495: The vicinity of its center. This process is also thought to explain why many barred spiral galaxies have active galactic nuclei , such as that seen in the Southern Pinwheel Galaxy . Bars are thought to be temporary phenomena in the lives of spiral galaxies; the bar structures decay over time, transforming galaxies from barred spirals to more "regular" spiral patterns. Past a certain size the accumulated mass of

8265-631: The white dwarf can explode as a Type Ia supernova if it approaches the Chandrasekhar limit . Occasionally, novae are bright enough and close enough to Earth to be conspicuous to the unaided eye. The brightest recent example was Nova Cygni 1975 . This nova appeared on 29 August 1975, in the constellation Cygnus about 5 degrees north of Deneb , and reached magnitude  2.0 (nearly as bright as Deneb). The most recent were V1280 Scorpii , which reached magnitude 3.7 on 17 February 2007, and Nova Delphini 2013 . Nova Centauri 2013

8360-529: The whole Andromeda Galaxy at about 2.5 × 10 ^  ly (1.6 × 10 AU). This new value is in excellent agreement with the previous, independent Cepheid-based distance value. The TRGB method was also used in 2005 giving a distance of 2.56 × 10 ^  ± 0.08 × 10 ^  ly (1.619 × 10 ± 5.1 × 10 AU). Averaged together, these distance estimates give a value of 2.54 × 10 ^  ± 0.11 × 10 ^  ly (1.606 × 10 ± 7.0 × 10 AU). Until 2018, mass estimates for

8455-413: Was V1369 Centauri , which reached 3.3 magnitude on 14 December 2013. During the sixteenth century, astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia . He described it in his book De nova stella ( Latin for "concerning the new star"), giving rise to the adoption of the name nova . In this work he argued that a nearby object should be seen to move relative to

8550-406: Was V445 Puppis , in 2000. Since then, four other novae have been proposed as helium novae. Astronomers have estimated that the Milky Way experiences roughly 25 to 75 novae per year. The number of novae actually observed in the Milky Way each year is much lower, about 10, probably because distant novae are obscured by gas and dust absorption. As of 2019, 407 probable novae had been recorded in

8645-467: Was deduced that Andromeda has a stellar nature. In 1885, a supernova (known as S Andromedae ) was seen in Andromeda, the first and so far only one observed in that galaxy. At the time, it was called "Nova 1885"—the difference between " novae " in the modern sense and supernovae was not yet known. Andromeda was considered to be a nearby object, and it was not realized that the "nova" was much brighter than ordinary novae. In 1888, Isaac Roberts took one of

8740-434: Was discovered 2 December 2013 and so far is the brightest nova of this millennium, reaching magnitude 3.3. A helium nova (undergoing a helium flash ) is a proposed category of nova event that lacks hydrogen lines in its spectrum . The absence of hydrogen lines may be caused by the explosion of a helium shell on a white dwarf. The theory was first proposed in 1989, and the first candidate helium nova to be observed

8835-506: Was hypothesized that the brighter portion of the double nucleus is the remnant of a small galaxy "cannibalized" by the Andromeda Galaxy, this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there

8930-588: Was long thought that the Andromeda Galaxy was more massive than the Milky Way by a margin of some 25% to 50%. However, this has been called into question by early 21st-century studies indicating a possibly lower mass for the Andromeda Galaxy and a higher mass for the Milky Way. The Andromeda Galaxy has a diameter of about 46.56 kpc (152,000 ly), making it the largest member of the Local Group of galaxies in terms of extension. The Milky Way and Andromeda galaxies are expected to collide with each other in around 4–5 billion years, merging to potentially form

9025-544: Was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity , naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way and elsewhere. (The existence of two distinct populations had been noted earlier by Jan Oort .) Baade also discovered that there were two types of Cepheid variable stars, which resulted in doubling

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