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41-401: The central pair is known as Rho Ophiuchi AB. It consists of at least two blue-colored subgiants or main-sequence stars , designated Rho Ophiuchi A and B, respectively. Rho Ophiuchi AB is a visual binary , and the sky-projected distance between the two stars appears to be 3.1″, corresponding to a separation of at least 344 astronomical units (au). However, the actual separation is larger, and

82-486: A continuum of stars between obvious main-sequence stars such as the Sun and obvious giant stars such as Aldebaran , although less numerous than either the main sequence or the giant stars. The Yerkes spectral classification system is a two-dimensional scheme that uses a letter and number combination to denote that temperature of a star (e.g. A5 or M1) and a Roman numeral to indicate the luminosity relative to other stars of

123-423: A degenerate helium core before this point and that will cause the star to enter the red giant branch as for lower mass stars. The core contraction and envelope expansion is very rapid, taking only a few million years. In this time the temperature of the star will cool from its main sequence value of 6,000–30,000 K to around 5,000 K. Relatively few stars are seen in this stage of their evolution and there

164-403: A larger helium core before leaving the main sequence, hence lower mass stars show a hook at the start of the subgiant branch. The helium core mass of a Z=0.001 (extreme population II ) 1  M ☉ star at the end of the main sequence is nearly double that of a Z=0.02 ( population I ) star. The low metallicity star is also over 1,000 K hotter and over twice as luminous at the start of

205-453: A pronounced subgiant branch in their color–magnitude diagrams . ω Centauri actually shows several separate subgiant branches for reasons that are still not fully understood, but appear to represent stellar populations of different ages within the cluster. Several types of variable star include subgiants: Subgiants more massive than the sun cross the Cepheid instability strip , called

246-418: A region above (i.e. more luminous than) the main sequence stars and below the giant stars. There are relatively few on most H–R diagrams because the time spent as a subgiant is much less than the time spent on the main sequence or as a giant star. Hot, class B, subgiants are barely distinguishable from the main sequence stars, while cooler subgiants fill a relatively large gap between cool main sequence stars and

287-463: A small low-mass companion. Related to this is its extremely strong magnetic field ; its dipole strength is at least B d = 1.9 ± 0.2 kG. Several other stars are located close to Rho Ophiuchi AB. HD 147932 is located 2.5 arcminutes away (at least 17,000 au), and is known as Rho Ophiuchi C. HD 147888 is located 2.82 arcminutes away (at least 19,000 au), and is known as Rho Ophiuchi DE. Stars C and D are both B-type main-sequence stars , and D itself

328-458: A stage known as the red-giant branch . The transition from the main sequence to the red giant branch is known as the subgiant branch. The shape and duration of the subgiant branch varies for stars of different masses, due to differences in the internal configuration of the star. Stars less massive than about 0.4  M ☉ are convective throughout most of the star. These stars continue to fuse hydrogen in their cores until essentially

369-406: A subgiant on its first crossing but was subsequently determined to be on its second crossing Planets in orbit around subgiant stars include Kappa Andromedae b , Kepler-36 b and c, TOI-4603 b and HD 224693 b . FK Comae Berenices FK Comae Berenices is a variable star that varies in apparent magnitude between 8.14 and 8.33 over a period of 2.4 days. It is the prototype for

410-408: Is a star that is brighter than a normal main-sequence star of the same spectral class , but not as bright as giant stars . The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star . The term subgiant was first used in 1930 for class G and early K stars with absolute magnitudes between +2.5 and +4. These were noted as being part of

451-401: Is a scatter plot of stars with temperature or spectral type on the x-axis and absolute magnitude or luminosity on the y-axis. H–R diagrams of all stars, show a clear diagonal main sequence band containing the majority of stars, a significant number of red giants (and white dwarfs if sufficiently faint stars are observed), with relatively few stars in other parts of the diagram. Subgiants occupy

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492-534: Is an apparent lack in the H–R diagram known as the Hertzsprung gap . It is most obvious in clusters from a few hundred million to a few billion years old. Beyond about 8–12  M ☉ , depending on metallicity, stars have hot massive convective cores on the main sequence due to CNO cycle fusion. Hydrogen shell fusion and subsequent core helium fusion begin quickly following core hydrogen exhaustion, before

533-560: Is another binary with an orbital period of around 680 years. Rho Ophiuchi is the namesake of the Rho Ophiuchi cloud complex . It is a nebula of gas and dust, which the Rho Ophiuchi system is embedded in. It is one of the easiest star forming regions to observe, as it is one of the nearest, and it is visible from both hemispheres. The interstellar extinction (A V ) of Rho Ophiuchi is measured to be 1.45 magnitudes , meaning

574-505: Is plotted for a group of stars which all have the same age, such as a cluster, the subgiant branch may be visible as a band of stars between the main sequence turnoff point and the red giant branch. The subgiant branch is only visible if the cluster is sufficiently old that 1–8  M ☉ stars have evolved away from the main sequence, which requires several billion years. Globular clusters such as ω Centauri and old open clusters such as M67 are sufficiently old that they show

615-541: Is still below the Schönberg–Chandrasekhar limit , but hydrogen shell fusion quickly increases the mass of the core beyond that limit. More-massive stars already have cores above the Schönberg–Chandrasekhar mass when they leave the main sequence. The exact initial mass at which stars will show a hook and at which they will leave the main sequence with cores above the Schönberg–Chandrasekhar limit depend on

656-418: Is unusually fast for a cool giant star . It is speculated that this is due to the merger of a contact binary pair of stars into a single star. The rotation produces extremely strong magnetic fields which are expected to brake the star to a slower rotation rate. Analysis of variability due to star spots on the surface show that the star rotates at different speeds at different latitudes. FK Comae Berenices

697-622: The FK Comae Berenices (FK Com) class of variable stars. The variability of FK Com stars may be caused by large, cool spots on the rotating surfaces of the stars. This star is thought to be the result of a recent binary merger, resulting in a high rate of both spin and magnetic activity . The spectral class of FK Comae Berenices is G4 III, although it is considered unusual in having very broad absorption lines as well as some emission lines . The broadened spectral lines are due to rapid rotation. The rotation rate of FK Comae Berenices

738-413: The first crossing since they may cross the strip again later on a blue loop . In the 2 – 3  M ☉ range, this includes Delta Scuti variables such as β Cas . At higher masses the stars would pulsate as Classical Cepheid variables while crossing the instability strip, but massive subgiant evolution is very rapid and it is difficult to detect examples. SV Vulpeculae has been proposed as

779-404: The central core continues to fuse without interruption. The star is considered to be a subgiant at this point although there is little change visible from the exterior. As the fusing hydrogen shell converts its mass into helium the convective effect separates the helium towards the core where it very slowly increases the mass of the non-fusing core of nearly pure helium plasma. As this takes place

820-407: The dust and gas in front of Rho Ophiuchi absorbs light from the system, making it appear 1.45 magnitudes dimmer than it would be if there were no dust or gas. Additionally, gas and dust also scatters more higher-frequency light, leaving the light appearing more reddish. The interstellar reddening ( E B−V ) of Rho Ophiuchi has been measured to be 0.47 magnitudes. Subgiant A subgiant

861-523: The entire star has been converted to helium, and they do not develop into subgiants. Stars of this mass have main-sequence lifetimes many times longer than the current age of the Universe. Stars with 40 percent the mass of the Sun and larger have non-convective cores with a strong temperature gradient from the centre outwards. When they exhaust hydrogen at the core of the star, the shell of hydrogen surrounding

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902-440: The envelope of the star and the luminosity stays approximately constant. The subgiant branch for these stars is short, horizontal, and heavily populated, as visible in very old clusters. After one to eight billion years, the helium core becomes too massive to support its own weight and becomes degenerate. Its temperature increases, the rate of fusion in the hydrogen shell increases, the outer layers become strongly convective, and

943-436: The fusing hydrogen shell gradually expands outward which increases the size of the outer shell of the star up to the subgiant size from two to ten times the original radius of the star when it was on the main sequence. The expansion of the outer layers of the star into the subgiant size nearly balances the increase energy generated by the hydrogen shell fusion causing the star to nearly maintain its surface temperature. This causes

984-570: The luminosity increases at approximately the same effective temperature. The star is now on the Red-giant branch . Stars as massive and larger than the Sun have a convective core on the main sequence. They develop a more massive helium core, taking up a larger fraction of the star, before they exhaust the hydrogen in the entire convective region. Fusion in the star ceases entirely and the core begins to contract and increase in temperature. The entire star contracts and increases in temperature, with

1025-488: The metallicity and the degree of overshooting in the convective core. Low metallicity causes the central part of even low mass cores to be convectively unstable, and overshooting causes the core to be larger when hydrogen becomes exhausted. Once the core exceeds the C–R limit, it can no longer remain in thermal equilibrium with the hydrogen shell. It contracts and the outer layers of the star expand and cool. The energy to expand

1066-426: The most useful spectral features for each spectral class are: Morgan and Keenan listed examples of stars in luminosity class IV when they established the two-dimensional classification scheme: Later analysis showed that some of these were blended spectra from double stars and some were variable, and the standards have been expanded to many more stars, but many of the original stars are still considered standards of

1107-400: The outer envelope causes the radiated luminosity to decrease. When the outer layers cool sufficiently, they become opaque and force convection to begin outside the fusing shell. The expansion stops and the radiated luminosity begins to increase, which is defined as the start of the red giant branch for these stars. Stars with an initial mass approximately 1–2  M ☉ can develop

1148-469: The primary star of the δ Circini system , both class O stars with masses of over 20  M ☉ . This table shows the typical lifetimes on the main sequence (MS) and subgiant branch (SB), as well as any hook duration between core hydrogen exhaustion and the onset of shell burning, for stars with different initial masses, all at solar metallicity (Z = 0.02). Also shown are the helium core mass, surface effective temperature, radius, and luminosity at

1189-449: The radiated luminosity actually increasing despite the lack of fusion. This continues for several million years before the core becomes hot enough to ignite hydrogen in a shell, which reverses the temperature and luminosity increase and the star starts to expand and cool. This hook is generally defined as the end of the main sequence and the start of the subgiant branch in these stars. The core of stars below about 2  M ☉

1230-607: The rate of fusion increases. This causes stars to evolve slowly to higher luminosities as they age and broadens the main sequence band in the Hertzsprung–Russell diagram . Once a main sequence star ceases to fuse hydrogen in its core, the core begins to collapse under its own weight. This causes it to increase in temperature and hydrogen fuses in a shell outside the core, which provides more energy than core hydrogen burning. Low- and intermediate-mass stars expand and cool until at about 5,000 K they begin to increase in luminosity in

1271-415: The red giants. Below approximately spectral type K3 the region between the main sequence and red giants is entirely empty, with no subgiants. Stellar evolutionary tracks can be plotted on an H–R diagram. For a particular mass, these trace the position of a star throughout its life, and show a track from the initial main sequence position, along the subgiant branch, to the giant branch. When an H–R diagram

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1312-454: The same temperature. Luminosity class IV stars are the subgiants, located between main-sequence stars (luminosity class V) and red giants (luminosity class III). Rather than defining absolute features, a typical approach to determining a spectral luminosity class is to compare similar spectra against standard stars. Many line ratios and profiles are sensitive to gravity, and therefore make useful luminosity indicators, but some of

1353-463: The spectral class of the star to change very little in the lower end of this range of star mass. The subgiant surface area radiating the energy is so much larger the potential circumstellar habitable zone where planetary orbits will be in the range to form liquid water is shifted much further out into any planetary system. The surface area of a sphere is found as 4πr so a sphere with a radius of 2  R ☉ will release 400% as much energy at

1394-487: The star could reach the red giant branch. Such stars, for example early B main sequence stars, experience a brief and shortened subgiant branch before becoming supergiants . They may also be assigned a giant spectral luminosity class during this transition. In very massive O-class main sequence stars, the transition from main sequence to giant to supergiant occurs over a very narrow range of temperature and luminosity, sometimes even before core hydrogen fusion has ended, and

1435-416: The start and end of the subgiant branch for each star. The end of the subgiant branch is defined to be when the core becomes degenerate or when the luminosity starts to increase. In general, stars with lower metallicity are smaller and hotter than stars with higher metallicity. For subgiants, this is complicated by different ages and core masses at the main sequence turnoff . Low metallicity stars develop

1476-400: The subgiant branch in a star like the sun are prolonged with little external indication of the internal changes. One approach to identifying evolutionary subgiants include chemical abundances such as Lithium which is depleted in subgiants, and coronal emission strength. As the fraction of hydrogen remaining in the core of a main sequence star decreases, the core temperature increases and so

1517-433: The subgiant branch. The difference in temperature is less pronounced at the end of the subgiant branch, but the low metallicity star is larger and nearly four times as luminous. Similar differences exist in the evolution of stars with other masses, and key values such as the mass of a star that will become a supergiant instead of reaching the red giant branch are lower at low metallicity. A Hertzsprung–Russell (H–R) diagram

1558-399: The subgiant class is rarely used. Values for the surface gravity, log(g), of O-class stars are around 3.6 cgs for giants and 3.9 for dwarfs. For comparison, typical log(g) values for K class stars are 1.59 ( Aldebaran ) and 4.37 ( α Centauri B ), leaving plenty of scope to classify subgiants such as η Cephei with log(g) of 3.47. Examples of massive subgiant stars include θ Orionis A and

1599-671: The subgiant luminosity class. O-class stars and stars cooler than K1 are rarely given subgiant luminosity classes. The subgiant branch is a stage in the evolution of low to intermediate mass stars. Stars with a subgiant spectral type are not always on the evolutionary subgiant branch, and vice versa. For example, the stars FK Com and 31 Com both lie in the Hertzsprung Gap and are likely evolutionary subgiants, but both are often assigned giant luminosity classes. The spectral classification can be influenced by metallicity, rotation, unusual chemical peculiarities, etc. The initial stages of

1640-404: The surface and a sphere with a 10  R ☉ will release 10000% as much energy. The helium core mass is below the Schönberg–Chandrasekhar limit and it remains in thermal equilibrium with the fusing hydrogen shell. Its mass continues to increase and the star very slowly expands as the hydrogen shell migrates outwards. Any increase in energy output from the shell goes into expanding

1681-408: The two take about 2,400 years to complete an orbit. The two stars dominate the radiation field around the Rho Ophiuchi cloud complex. Rho Ophiuchi A emits X-rays , and exhibits strong variability in emission over periods of about 1.2 days, corresponding to its rotation period . The exact origin of its X-ray variability is unknown: it could be an magnetically active spot on its surface, or it could be

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