Astronomers have found the first green star and one of the reddest white dwarfs.
- Новости
- Science and technology
- Astronomers have found the first green star and one of the reddest white dwarfs.
Stars can have not only the usual white, yellow and blue shades, but also green, pink and purple. Astronomers have found that the unusual colors of some stellar remnants are related to the chemical composition and high density of their atmospheres. About how scientists discovered a green white dwarf and why another object looks pink-red, in the Izvestia article.
How scientists discovered stars of unusual colors
Astronomers have discovered a white dwarf that, when viewed at close range, could look green, as well as an object with an unusual rich pink-red hue. The results of the study are published in the RAS Techniques and Instruments journal. The work shows that the color of white dwarfs is determined not only by temperature, but also by the characteristics of their atmospheres.
White dwarfs are dense remnants of stars like the Sun that have exhausted their thermonuclear fuel reserves and shed their outer layers. They continue to emit light, gradually cooling down.
Previously, astronomers usually associated their color with temperature: hotter objects look white or bluish, while relatively cool ones look orange. However, new work shows that chemical elements in the atmosphere can significantly change this color.
Mark Hollands, an astronomer at the University of Warwick and co-author of the study
Most stars, including most white dwarfs, behave like "ideal radiators," so their color is almost entirely determined by temperature. But no one has really explored what other options are possible for white dwarfs. It turns out that these incredibly dense stars can produce colors that no one expected, and some of them really surprised me.
The scientists did not see these colors directly through the telescope. White dwarfs are too small and dim for the human eye to discern their color at such a distance. The researchers studied the spectra of stars — the distribution of their radiation by wavelength — and processed the data using a model of human color perception. This made it possible to calculate what the light of each object would look like to a person who was close enough.
Why white dwarfs can be green and pink
The color of most stars is related to their surface temperature. However, the atmospheres of the coldest white dwarfs are extremely dense.: It can approach the density of water and be many times higher than the density of the outer layers of the Sun's atmosphere. In such conditions, chemical elements interact with the surrounding helium differently than in a less dense environment and absorb light in wider wavelength ranges.
As a result, the spectrum of the star changes, and with it the hue that the human eye perceives. The essential role is played by elements present in the atmosphere in small quantities: for example, sodium, calcium and carbon. Their influence can be so strong that the color of the white dwarf ceases to correspond to what would be expected based on its temperature alone.
The chemical composition of the atmosphere also depends on the history of the star. Some white dwarfs attract fragments of asteroids and other rocky bodies from their former planetary system. As a result, elements including calcium, magnesium, sodium and lithium accumulate in their atmosphere. In other objects, convective flows lift carbon from the depths to the surface, where it forms molecules that absorb visible light.
Therefore, in computational models, white dwarfs can look yellow, green, pink, purple, or bluish-turquoise. This does not mean that stars glow in the same way as colored objects: we are talking about the color of their radiation, which is determined by the spectrum and features of human vision.
A green white dwarf and an unusually red star
One of the most unusual objects is the white dwarf WD J1330+6435, located about 284 light-years from Earth. Its temperature is about 4 thousand degrees. The star attracted rocky matter from its planetary system, so calcium, lithium and sodium are present in its atmosphere.
Sodium plays a particularly important role: in high-density conditions, it absorbs light in a wide range of the spectrum, including a significant portion of yellow, orange and red wavelengths. Calcium, in turn, absorbs light in the purple area. As a result, the maximum radiation shifts to the green part of the visible spectrum, and in the calculated model WD J1330+6435 acquires a yellowish-green hue, which scientists call chartreuse.
Mark Hollands
Traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving a ghostly green glow. Imagine that you are standing on a planet and looking at a green star. It looks like something out of science fiction.
Another unusual object is the white dwarf WD J0902+2010, located about 48 light-years from Earth. Its deep convective zone reaches the core of the star and lifts carbon from the inner layers to the surface. Carbon molecules are formed there, which absorb light in a wide range of the visible spectrum, including blue, green and yellow wavelengths. As a result, the star looks pink with a rich salmon hue and may turn out to be one of the reddest known stars.
How rare are unusually colored stars
The researchers also analyzed a sample of 152 white dwarfs within about 65 light-years of the Sun. Five objects had unusual shades: two looked yellow, and three looked pink. Thus, a noticeably different coloration from the usual one was found in about one of the 30 studied objects. However, this estimate applies to a specific sample and does not mean that the same proportion is typical for all white dwarfs in the universe.
The other 147 stars had a median color temperature of about 6.8 thousand kelvin. This value describes the color character of the radiation and does not necessarily match the actual surface temperature. The result corresponds to a predominantly white color. This is consistent with the usual idea of white dwarfs, although individual objects can look completely different.
The researchers note that previous observations may have created a distorted view of the colors of white dwarfs. Hotter and bluer stars are usually brighter and therefore visible at greater distances. Because of this, they may be disproportionately represented in brightness-based samples. Studying nearby objects helps to accurately assess the variety of colors of this group of stars.
Boris Gansicke, an astronomer at the University of Warwick and co-author of the study
One out of 30 may not be such a large proportion, but in the totality of white dwarfs in the universe, this is still a huge number of stars with unusual colors. However, if we take the median for a representative population of white dwarfs, we find that the average object is indeed white.
The work shows that the spectra of stars contain more information about their atmospheres than can be assumed from temperature alone. The unusual coloration helps to study the chemical composition of stellar remnants and the processes occurring in their outer layers. At the same time, the calculated colors depend on the quality of spectral data and models, therefore, refinements are possible for individual objects as new observations become available.
Переведено сервисом «Яндекс Переводчик»
