A red giant is a luminous
giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moo ...
of low or intermediate mass (roughly 0.3–8
solar mass
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
es ()) in a late phase of
stellar evolution
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...
. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or lower. The appearance of the red giant is from yellow-white to reddish-orange, including the
spectral types K and M, sometimes G, but also
class S stars and most
carbon star
A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes mos ...
s.
Red giants vary in the way by which they generate energy:
* most common red giants are stars on the
red-giant branch
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass sta ...
(RGB) that are still
fusing hydrogen into helium in a shell surrounding an inert helium core
*
red-clump stars in the cool half of the
horizontal branch
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) ...
, fusing helium into carbon in their cores via the
triple-alpha process
The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.
Triple-alpha process in stars
Helium accumulates in the cores of stars as a result of the proton–pro ...
*
asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen-burning shell just beyond that.
Many of the well-known bright stars are red giants because they are luminous and moderately common. The K0 RGB star
Arcturus
, - bgcolor="#FFFAFA"
, Note (category: variability): , , H and K emission vary.
Arcturus is the brightest star in the northern constellation of Boötes. With an apparent visual magnitude of −0.05, it is the third-brightest of th ...
is 36
light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s away, and
Gamma Crucis
Gacrux it is the third-brightest star in the southern constellation of Crux, the Southern Cross. It has the Bayer designation Gamma Crucis, which is Latinised from γ Crucis and abbreviated Gamma Cru or γ Cru. With an apparent visual ma ...
is the nearest M-class giant at 88 light-years' distance.
A red giant will usually produce a
planetary nebula
A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
The term "planetary nebula" is a misnomer because they are unrelated to ...
and become a
white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
at the end of its life.
Characteristics
![Structure of Stars (artist’s impression)](https://upload.wikimedia.org/wikipedia/commons/c/c3/Structure_of_Stars_%28artist%E2%80%99s_impression%29.jpg)
A red giant is a star that has exhausted the supply of hydrogen in its core and has begun
thermonuclear fusion
Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the re ...
of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. However, their outer envelope is lower in temperature, giving them a yellowish-orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun (), spectral types of K or M, have surface temperatures of 3,000–4,000 K, and radii up to about 200 times the Sun (). Stars on the horizontal branch are hotter, with only a small range of luminosities around . Asymptotic-giant-branch stars range from similar luminosities as the brighter stars of the red-giant branch, up to several times more luminous at the end of the thermal pulsing phase.
Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements are convected to the surface in what is called a
dredge-up A dredge-up is any one of several stages in the evolution of some stars. By definition, during a ''dredge-up'', a convection zone extends all the way from the star's surface down to the layers of material that have undergone fusion. Consequently, th ...
.
The first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, and sometimes third, dredge up occurs during helium shell burning on the asymptotic-giant branch and convects carbon to the surface in sufficiently massive stars.
The stellar limb of a red giant is not sharply defined, contrary to their depiction in many illustrations. Rather, due to the very low mass density of the envelope, such stars lack a well-defined
photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
, and the body of the star gradually transitions into a '
corona
Corona (from the Latin for 'crown') most commonly refers to:
* Stellar corona, the outer atmosphere of the Sun or another star
* Corona (beer), a Mexican beer
* Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
'.
The coolest red giants have complex spectra, with molecular lines, emission features, and sometimes masers, particularly from thermally pulsing AGB stars.
Observations have also provided evidence of a hot chromosphere above the photosphere of red giants, where investigating the heating mechanisms for the chromospheres to form requires 3D simulations of red giants.
Another noteworthy feature of red giants is that, unlike Sun-like stars whose photospheres have a large number of small convection cells (
solar granules), red-giant photospheres, as well as those of
red supergiant
Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Anta ...
s, have just a few large cells, the features of which cause the
variations of brightness so common on both types of stars.
[
]
Evolution
![The life cycle of a Sun-like star (annotated)](https://upload.wikimedia.org/wikipedia/commons/6/6f/The_life_cycle_of_a_Sun-like_star_%28annotated%29.jpg)
Red giants are evolved from
main-sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...
stars with masses in the range from about to around .
[ When a star initially ]forms
Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens.
Form also refers to:
*Form (document), a document (printed or electronic) with spaces in which to write or enter data
* ...
from a collapsing molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen ...
in the interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
, it contains primarily hydrogen and helium, with trace amounts of "metals
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
" (in stellar structure, this simply refers to ''any'' element that is not hydrogen or helium i.e. atomic number
The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
greater than 2). These elements are all uniformly mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (a few million kelvin) and establishes hydrostatic equilibrium
In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ...
. Over its main sequence life, the star slowly converts the hydrogen in the core into helium; its main-sequence life ends when nearly all the hydrogen in the core has been fused. For the Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, the main-sequence lifetime is approximately 10 billion years. More-massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars.
When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and so the core begins to contract due to its gravity. This brings additional hydrogen into a zone where the temperature and pressure are sufficient to cause fusion to resume in a shell around the core. The hydrogen-burning shell results in a situation that has been described as the ''mirror principle''; when the core within the shell contracts, the layers of the star outside the shell must expand. The detailed physical processes that cause this are complex. Still, the behavior is necessary to satisfy simultaneous conservation of gravitational
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
and thermal energy
The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, d ...
in a star with the shell structure. The core contracts and heats up due to the lack of fusion, and so the outer layers of the star expand greatly, absorbing most of the extra energy from shell fusion. This process of cooling and expanding is the subgiant
A subgiant 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 ...
star. When the envelope of the star cools sufficiently it becomes convective, the star stops expanding, its luminosity starts to increase, and the star is ascending the red-giant branch
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass sta ...
of the Hertzsprung–Russell (H–R) diagram.[
]
The evolutionary path the star takes as it moves along the red-giant branch depends on the mass of the star. For the Sun and stars of less than about the core will become dense enough that electron degeneracy pressure
Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant pressure in addition to, or in lieu of, thermal pressure. The description applies to matter composed of electrons, protons, neu ...
will prevent it from collapsing further. Once the core is degenerate
Degeneracy, degenerate, or degeneration may refer to:
Arts and entertainment
* Degenerate (album), ''Degenerate'' (album), a 2010 album by the British band Trigger the Bloodshed
* Degenerate art, a term adopted in the 1920s by the Nazi Party i ...
, it will continue to heat until it reaches a temperature of roughly 108 K, hot enough to begin fusing helium to carbon via the triple-alpha process
The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.
Triple-alpha process in stars
Helium accumulates in the cores of stars as a result of the proton–pro ...
. Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash
A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses () and 2.0 ) during their red giant phase (the Sun is ...
. In more-massive stars, the collapsing core will reach 108 K before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, and produce no helium flash.[ The core helium fusing phase of a star's life is called the ]horizontal branch
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) ...
in metal-poor stars, so named because these stars lie on a nearly horizontal line in the H–R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump
Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondary ...
in the H–R diagram.
An analogous process occurs when the central helium is exhausted and the star collapses once again, causing helium in a shell to begin fusing. At the same time, hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...
, a second red-giant phase. The helium fusion results in the build-up of a carbon–oxygen core. A star below about will never start fusion in its degenerate carbon–oxygen core.[ Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a ]planetary nebula
A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
The term "planetary nebula" is a misnomer because they are unrelated to ...
with the core of the star exposed, ultimately becoming a white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
. The ejection of the outer mass and the creation of a planetary nebula finally ends the red-giant phase of the star's evolution.[ The red-giant phase typically lasts only around a billion years in total for a solar mass star, almost all of which is spent on the red-giant branch. The horizontal-branch and asymptotic-giant-branch phases proceed tens of times faster.
If the star has about 0.2 to ,][ it is massive enough to become a red giant but does not have enough mass to initiate the fusion of helium.][ These "intermediate" stars cool somewhat and increase their luminosity but never achieve the tip of the red-giant branch and helium core flash. When the ascent of the red-giant branch ends they puff off their outer layers much like a post-asymptotic-giant-branch star and then become a white dwarf.
]
Stars that do not become red giants
Very-low-mass stars are fully convective and may continue to fuse hydrogen into helium for up to a trillion years until only a small fraction of the entire star is hydrogen. Luminosity and temperature steadily increase during this time, just as for more-massive main-sequence stars, but the length of time involved means that the temperature eventually increases by about 50% and the luminosity by around 10 times. Eventually the level of helium increases to the point where the star ceases to be fully convective and the remaining hydrogen locked in the core is consumed in only a few billion more years. Depending on mass, the temperature and luminosity continue to increase for a time during hydrogen shell burning, the star can become hotter than the Sun and tens of times more luminous than when it formed although still not as luminous as the Sun. After some billions more years, they start to become less luminous and cooler even though hydrogen shell burning continues. These become cool helium white dwarfs.
Very-high-mass stars develop into supergiant
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...
s that follow an evolutionary track
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...
that takes them back and forth horizontally over the H–R diagram, at the right end constituting red supergiant
Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Anta ...
s. These usually end their life as a type II supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
. The most massive stars can become Wolf–Rayet star
Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface ...
s without becoming giants or supergiants at all.
Planets
Red giants with known planets: the M-type HD 208527
HD 208527 is a star located in the northern constellation of Pegasus. It has a reddish hue and is dimly visible to the naked eye with an apparent visual magnitude of +6.39. The star is located at a distance of approximately 1,300 ...
, HD 220074
HD 220074 is a star located in the northern constellation of Cassiopeia, near the western border with Cepheus. It has a reddish hue and is dimly visible to the naked eye, having an apparent visual magnitude of +6.39. The star is lo ...
and, as of February 2014, a few tens of known K-giants including Pollux, Gamma Cephei
Gamma Cephei (γ Cephei, abbreviated Gamma Cep, γ Cep) is a binary star system approximately 45 light-years away in the constellation of Cepheus. The primary (designated Gamma Cephei A, officially named Errai , the traditional name of ...
and Iota Draconis
Iota Draconis (ι Draconis, abbreviated Iota Dra, ι Dra), also named Edasich , is a star in the northern circumpolar constellation of Draco. A visually unremarkable star of apparent magnitude 3.3, in 2002 it was discovered to have a pl ...
.
Prospects for habitability
Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system
A planetary system is a set of gravitationally
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interacti ...
, if present, uninhabitable, some research suggests that, during the evolution of a star along the red-giant branch, it could harbor a habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
for several billion years at 2 astronomical unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
s (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. After the red-giant stage, there would for such a star be a habitable zone between 7 and 22 AU for an additional one billion years. Later studies have refined this scenario, showing how for a star the habitable zone lasts from 100 million years for a planet with an orbit similar to that of Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
to 210 million years for one that orbits at Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
's distance to the Sun, the maximum time (370 million years) corresponding for planets orbiting at the distance of Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
. However, planets orbiting a star in equivalent orbits to those of Jupiter and Saturn would be in the habitable zone for 5.8 billion years and 2.1 billion years, respectively; for stars more massive than the Sun, the times are considerably shorter.
Enlargement of planets
As of June 2014, fifty giant planets have been discovered around giant stars. However, these giant planets are more massive than the giant planets found around solar-type stars. This could be because giant stars are more massive than the Sun (less massive stars will still be on the main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
and will not have become giants yet) and more massive stars are expected to have more massive planets. However, the masses of the planets that have been found around giant stars do not correlate with the masses of the stars; therefore, the planets could be growing in mass during the stars' red giant phase. The growth in planet mass could be partly due to accretion from stellar wind, although a much larger effect would be Roche lobe
In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, wit ...
overflow causing mass-transfer from the star to the planet when the giant expands out to the orbital distance of the planet.
Well known examples
Many of the well-known bright stars are red giants, because they are luminous and moderately common. The red-giant branch variable star Gamma Crucis
Gacrux it is the third-brightest star in the southern constellation of Crux, the Southern Cross. It has the Bayer designation Gamma Crucis, which is Latinised from γ Crucis and abbreviated Gamma Cru or γ Cru. With an apparent visual ma ...
is the nearest M-class giant star at 88 light-years. The K1.5 red-giant branch star Arcturus
, - bgcolor="#FFFAFA"
, Note (category: variability): , , H and K emission vary.
Arcturus is the brightest star in the northern constellation of Boötes. With an apparent visual magnitude of −0.05, it is the third-brightest of th ...
is 36 light-years away.
Red-giant branch
* Aldebaran
Aldebaran (Arabic: “The Follower”, "الدبران") is the brightest star in the zodiac constellation of Taurus. It has the Bayer designation α Tauri, which is Latinized to Alpha Tauri and abbreviated Alpha Tau or α Tau. Aldebar ...
(α Tauri)
* Arcturus
, - bgcolor="#FFFAFA"
, Note (category: variability): , , H and K emission vary.
Arcturus is the brightest star in the northern constellation of Boötes. With an apparent visual magnitude of −0.05, it is the third-brightest of th ...
(α Bootis)
* Gacrux
Gacrux it is the third-brightest star in the southern constellation of Crux, the Southern Cross. It has the Bayer designation Gamma Crucis, which is Latinised from γ Crucis and abbreviated Gamma Cru or γ Cru. With an apparent visual ma ...
(γ Crucis)
Red-clump giants
* Hamal
Hamal , designation Alpha Arietis (α Arietis, abbreviated Alpha Ari, α Ari), is the brightest star in the northern zodiacal constellation of Aries.
With an apparent visual magnitude of 2.0, it is the mean 50th-brightest star in the n ...
(α Arietis)
* κ Persei
* δ Andromedae
Asymptotic giant branch
* Mira
Mira (), designation Omicron Ceti (ο Ceti, abbreviated Omicron Cet, ο Cet), is a red-giant star estimated to be 200–400 light-years from the Sun in the constellation Cetus.
ο Ceti is a binary stellar system, consisting of a varia ...
(ο Ceti)
* χ Cygni
* α Herculis
Alpha Herculis (α Herculis, abbreviated Alpha Her, α Her), also designated 64 Herculis, is a multiple star system in the constellation of Hercules. Appearing as a single point of light to the naked eye, it is resolvable into a number ...
The Sun as a red giant
The Sun will exit the main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
in approximately 5 billion years and start to turn into a red giant. As a red giant, the Sun will grow so large that it will engulf Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
, Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
, and possibly Earth, maybe even Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
and part or all of the asteroid belt.
References
External links
{{Use dmy dates, date=July 2019
Star types