While red giant stars are among the biggest stars in the Universe, they were not born that way. Red giants are for the most part normal main-sequence stars that have exhausted their supply of hydrogen, which initiates a process that causes their outer layers to expand hugely, while their surface temperatures decrease to as low as 5,000K, and sometimes lower.
Red giant stars can also have a range of colors from yellow-orange to various shades of red, and the term “red giant” typically includes stars of the spectral classes K and M, but also S-class stars, and most carbon stars. In general terms, red giants stars represent the late evolutionary stage of low-mass stars, whereas red supergiants stars are the late evolutionary stage of high-mass stars. Below are 10 more interesting facts about red giant stars you may not have known.
VY Canis Majoris (VY CMa) is the biggest known red giant star
Since red giant stars come in a range of diameters and masses, different types of red giants are placed on different locations on the H-R diagram, as the image below shows. Note the position where supergiant red giants, also sometimes known as “hyper giants” live. One such star is VY Canis Majoris, in the constellation Canis Major, which was originally a normal, main sequence O-type star. In terms of size, the star is 1,420 times bigger than the Sun, and has about 17 ± 8 solar masses.
At a distance of about 3,900 light years away, VY Canis Majori cannot be seen without optical aid, but studies have shown that it shines with a total luminosity somewhere between 250,000 and 500,00 times that of the Sun, making it one of the most luminous red giants known.
Most red giant stars are still evolving
Most red giant stars are still fusing hydrogen into helium, albeit not in their cores. Evolving red giants fuse hydrogen into helium in a shell that surrounds their inert helium cores, but various processes, that mainly depend on the star’s mass, can cause some red giants to switch back and forth between giant and super giant phases, as shown by the blue tracks in the image above.
Red giant stars do not have sharply defined limbs
Contrary to many depictions, red giants do not have clear limbs or “edges” because of the fact that their outer layers are exceedingly tenuous. Instead, the main body of red giants generally thin out with distance from their denser layers, with the outer parts of the star progressively transitioning into a kind of corona.
Not all main sequence stars can become red supergiants
Main sequence stars with about 30% to 800% solar mass typically evolve into red giants, but stars with between 20% and 50% of the Sun’s mass do not have sufficient mass as to initiate helium burning in their “normal” red giant phase. These stars typically remain in the red giant phase for about a billion years before blowing off their outer layers to form planetary nebulae, while their cores transition into white dwarfs that can take several trillion years to cool down.
Red giant stars have giant planets
While giant stars are expected to host giant planets, the masses of the 50 or so giant planets around red giants discovered thus far do not correlate with the masses of their host stars. One possible explanation is that when red giants expand, their planets accrete some stellar material when this material comes within reach of the planets’ gravitational fields, in a process known as the Roche lobe overflow mechanism.
Red giant stars may support life
While a one solar mass star may destroy a planetary system during its evolution into a red giant, new studies suggest that a Sun-like (one solar mass) star may support a habitable zone for several billion years at a distance of about 2 astronomical units, out to about 9 astronomical units while it is evolving into a red giant. Moreover, after our hypothetical one solar mass star has fully evolved into a red giant, it could easily support a habitable zone at between 7 and 22 astronomical units for an additional one billion years.
Earth may yet survive the Sun as a red giant
While it is certain that the Sun will evolve into a red giant, there is some uncertainty as to whether or not its outer layers will envelop the Earth. One factor in this equation is the fact that red giants are known to lose mass at a prodigious rate due to the weak hold gravity has over its outer layers. If current models are correct, the rate at which the Sun will lose a lot of mass will cause Earth’s orbital distance to increase, and some investigators believe Earth will move far enough away to escape the worst effects of the swollen Sun.
Most red giant stars are variable
Unlike Sun-like stars that have huge numbers of smallish convection cells in their photospheres, red giant stars, as well as red supergiant stars have only a few very big convection cells. Due to the differences in how convection cells transport heat (and light) between red giants and Sun-like stars, the large convection cells cause relatively big variations in the brightness of red giant stars when compared to the brightness variations in say, the Sun.
Red supergiant stars hardly rotate
All red supergiant stars observed so far rotate either slowly, or very slowly, and in some cases, it is difficult to determine if the star is rotating at all. This is due to the effects of mass loss, a process that effectively brakes a star’s rotation. In a few cases, such as the star Betelgeuse which rotates at only about 5 km/sec, the rotation may derive from binary interactions with other stars. However, the cores of red giants still rotate and do so at rates that far exceed the rotation of the star’s outer regions.
Many red giant stars are invisible at optical wavelengths
One particular type of red giant star, called asymptotic red giants, are so active and unstable that the rate at which they expel enormous amounts of their own matter soon enshrouds them in a dense cocoon (not to be confused with a planetary nebula) of dust and condensed stellar material that effectively renders these stars invisible to the wavelengths of light that human vision can perceive.
While the main mechanisms that drive or cause the huge rate of mass loss in these stars are not yet fully understood, it is thought that radiation pressure and violent pulsations in these stars are the main drivers.