Since time immemorial, we’ve wondered, “Is the Sun just a typical star?”
From their earliest beginnings to their maximum before fading away, Sun-like stars will grow from their current size to that of a red giant (~Earth orbit) to a diameter of about 5 light-years, typically. The largest known planetary nebulae can be nearly twice that size, up to about 10 light-years across, but none of this necessarily means that the Sun is an ordinary, average star.
In the 17th century, Christiaan Huygens estimated the distance to Sirius, assuming that it was a distant sun-like star.
Sirius A and B, a star bluer and brighter than our Sun and a white dwarf star, respectively, as imaged by the Hubble Space Telescope. Sirius A is the brightest star in the sky, but preliminary estimates of its distance were low, because they did not take into account the fact that Sirius is about 20 times as bright as our Sun.
His result, 0.4 light-years across, did not take into account intrinsic stellar differences.
The (modern) Morgan-Keenan spectral classification system, with the temperature range of each star class shown above, in Kelvin. The vast majority (80%) of stars today are M-class stars, with only 1 in 800 O- or B-class stars being massive enough to collapse as a supernova. Our Sun is a G-class star, unremarkable but brighter than all but about 5% of the stars. Only about half of the stars exist in isolation; The other half is associated with multiple star systems.
Stars come with a variety of properties: mass, color, temperature, ionization, metallicity, age, and so on.
This part of the Hubble image of Arp 143 shows the new stars (in blue) that formed as a result of gas being stripped, heated, and rammed into the space between the two main galactic elements. Stars have been forming all over the universe for the past 13.6 billion years or so, but the stars that survive today did not form evenly or under the same conditions over the course of cosmic history.
Although the Sun is not a unique cosmic exception, it is not entirely typical either.
Over the course of 50 days, with a total of more than 2 million seconds of total observation time (equivalent to 23 full days), the Hubble Deep Field (XDF) was created from part of a previous Hubble Ultra-Deep Field image. By combining light from ultraviolet through visible light and beyond to Hubble’s near infrared, the XDF represented humanity’s deepest view of the universe: a record that held out until it was broken by JWST. In the red square, where Hubble sees no galaxies, JWST’s JADES survey revealed the most distant galaxy yet: JADES-GS-z13-0. Extrapolating beyond what we see to what we know and expect to exist, we deduce a total of about 2 sextillion stars within the visible universe.
With about two sextilions (~2 x 1021) Stars within the visible universe, how do they compare?
The rate of star formation in the universe as a function of redshift, which is itself a function of cosmic time. The overall rate (left) is derived from both ultraviolet and infrared observations, and is remarkably consistent across time and space. Note that star formation, today, is only a few percent of what it was at its peak, and that the vast majority of stars formed in the first 4-5 billion years of our cosmic history. Only about 15% of all stars, at most, formed in the past 4.6 billion years.
Most of the stars that exist today formed long ago: ~11 billion years in the past.
This glimpse of stars in the densest region of the Orion Nebula, near the heart of the Trapezium Cluster, provides a recent glimpse into the Milky Way’s star-forming region. However, the characteristics of star formation vary over cosmic time, from one galaxy to another, at different radii from the galactic center, and so on. All of these characteristics and more must be calculated to compare the Sun with the total number of stars within the Universe.
Our Sun, which was born 4.6 billion years ago, is younger than 85% of all stars.
There are many galaxies comparable to the present Milky Way through cosmic time, as they have grown in mass and with a more developed structure at the present time. Smaller galaxies are inherently smaller, bluer, messier, richer in gas, have lower densities of heavy elements than their modern-day counterparts, and their history of star formation has evolved over time. Most of the stars in the universe formed disproportionately long ago, rather than relatively recently.
The majority of stars are red dwarfs: cool, low-mass, and long-lived.
This image shows the closest star system to Earth: Alpha Centauri. The bright star towards the left of the image is both Alpha Centauri A and Alpha Centauri B, which cannot be divided into two stars using most modern telescopes, while Proxima Centauri is very faint and circled in red. This is currently the closest star system to Earth; Proxima Centauri is a red dwarf, like about 75-80% of all stars, but it differs significantly from a less common star such as the Sun or Alpha Centauri A.
Our Sun, a G-magnitude star, is more than 95% larger than the stars.
A Hubble view of the globular cluster Terzan 5, just 22,000 light-years away in our Milky Way galaxy, reveals its bright core and stars of a wide variety of colors and masses. As gorgeous as the 2022 Hubble image is, the brightest stars within are the largest evolving giants and the largest surviving stars. The majority of stars are dim and low in mass, and can hardly be seen in an image like this at all.
Most of the stars are less metallic than our stars: some of the heavy elements are present.
This color-coded map shows the heavy element abundances of more than 6 million stars within the Milky Way. The red, orange, and yellow stars are all rich enough in heavy elements that they must have planets. Stars coded green and blue rarely have planets, and stars coded blue or violet should not have planets at all. Note that the central plane of the galactic disk, extending all the way to the galactic core, has the potential for habitable rocky planets.
Our Sun has a greater enrichment than ~93% of all stars.
These graphs show the estimated star formation rate intensity as a function of the redshift and metallicity of the stars that are forming. Although there are significant uncertainties, it can be safely concluded that only somewhere between 3% and 20% of all stars contain a heavy element greater than or equal to our Sun, with most estimates falling between only 4-10%.
Only half of the stars are as single as our Sun. The other half is within multiple star systems.
Although planets in triple systems have been found before in recent years, most orbit either close to one star or in intermediate orbits around a central binary, with the third star very far away. GW Orionis is the first candidate system to have a planet orbiting all three stars simultaneously. About 35% of all stars are in binary systems and another 10% are in triple systems; Only about half of the stars are as single as our Sun.
We are not usually luminous.
When a star-forming region becomes so large that it spans an entire galaxy, that galaxy becomes a star galaxy. Here, Henize 2-10 is shown evolving towards that state, with young stars in many locations and active stellar nurseries in many locations across the galaxy. If we counted the number of stars within the galaxy and multiplied that number by the light-to-mass ratio of the Sun, we would reduce the total flux by about 3 to 1.
The ratio of luminosity to the total mass of stars is three times that of ours.
Brown dwarfs, between about 0.013-0.080 solar masses, will fuse deuterium + deuterium into helium-3 or tritium, remaining roughly the same size as Jupiter but achieving much larger masses. Red dwarfs are only slightly larger, but even the sunlike star shown here doesn’t show scale here; Its diameter would be about 7 times that of a low-mass star.
Ordinary, it would seem, includes a colossal scope.
This Wolf-Rayet star is known as WR 31a, located about 30,000 light-years away in the constellation Carina. The outer nebula is expelled from hydrogen and helium, while the central star burns at more than 100,000 K. In the relatively near future, this star will explode in a supernova, enriching the surrounding interstellar medium with new heavy elements. With the exception of lower-mass stars, the hydrogen-rich outer layers of stars will be ejected back into the interstellar medium when nuclear fusion in the star’s core stops. Although Wolf-Rayet stars are rare, they are within the “normal” range for a star.
Mostly Mute Monday tells an astronomical story with pictures and visuals and no more than 200 words. taciturn; smile more.