The Sun isn’t a typical star in the Universe

Since time immemorial, we’ve wondered, “Is the Sun just a typical star?”

From their earliest beginnings to their final extent before fading away, Sun-like stars will grow from their present size to the size of a red giant (~the Earth’s orbit) to up to ~5 light-years in diameter, typically. The largest known planetary nebulae can reach approximately double that size, up to ~10 light-years across, but none of this necessarily means that the Sun is a typical, average star.

In the 1600s, Christiaan Huygens estimated the distance to Sirius, assuming it was a distant, Sun-like star.

This image shows Sirius A and B, a bluer and brighter star than our Sun and a white dwarf star, respectively, as imaged by the Hubble space telescope. Sirius A, the main star, is an A class star (as opposed to our Sun being a G class star): twice as massive as the Sun, some ~4000 K hotter than the Sun at its photosphere, and about 25 times as intrinsically luminous as our Sun. Sirius B once had about five times the Sun’s mass, but is now less massive, as a white dwarf, than its surviving stellar companion.

His result, 0.4 light-years, didn’t account for intrinsic stellar differences.

The (modern) Morgan–Keenan spectral classification system, with the surface temperature range of each star class shown above it, in kelvin. The overwhelming majority of stars today are M-class stars, with only 1 known O- or B-class star within 25 parsecs. Our Sun is a G-class star, along with about 5-10% of total stars. However, in the early Universe, almost all of the stars were O- or B-class stars, with an average mass 25 times greater than average stars today.

Stars come with a variety of properties: mass, color, temperature, ionization, metallicity, age, etc.

This portion of the Hubble image of Arp 143 showcases the new stars (in blue) formed as a result from gas stripping, heating, and shocking in the space between the two main galaxy members. Stars have been forming throughout the Universe over the past 13.6 billion years or so, but the ones that survive today weren’t formed evenly or under the same conditions over all of cosmic history.

Although the Sun isn’t a unique cosmic outlier, it isn’t exactly typical, either.

Over the course of 50 days, with a total of over 2 million seconds of total observing time (the equivalent of 23 complete days), the Hubble eXtreme Deep Field (XDF) was constructed from a portion of the prior Hubble Ultra Deep Field image. Combining light from ultraviolet through visible light and out to Hubble’s near-infrared limit, the XDF represented humanity’s deepest view of the cosmos: a record that stood until it was broken by JWST. In the red box, where no galaxies are seen by Hubble, the JWST’s JADES survey revealed the most distant galaxy to date: JADES-GS-z13-0. Extrapolating beyond what we see to what we know and expect must exist, we infer a total of ~2 sextillion stars within the observable Universe.

With around two sextillion (~2 × 10²¹) stars within the observable Universe, how do we compare?

The star-formation rate in the Universe is 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 (between 3-5%), and that the majority of stars were formed in the first ~5 billion years of our cosmic history. Only about ~15% of all stars, at maximum, have formed over the past 4.6 billion years. Direct measures of star-formation are important, but the method of Fermi-LAT for measuring the total number of photons produced by stars is superior.

Most stars that exist today formed long ago: ~11 billion years in the past.

This glimpse into the stars found in the densest region of the Orion Nebula, near the heart of the Trapezium Cluster, shows a modern glimpse inside a star-forming region of the Milky Way. However, star-formation properties vary over cosmic time, from galaxy to galaxy, at different radii from the galactic center, etc. All of these properties and more must be reckoned with to compare the Sun with the overall population of stars within the Universe. Note that our Sun, born 4.6 billion years ago, is younger than 85% of all stars.

Our Sun, born 4.6 billion years ago, is younger than 85% of all stars.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, and richer in gas in general than the galaxies we see today. Fewer galaxies have disks and spiral shapes as we look farther back in time. Over time, many smaller galaxies become gravitationally bound together, resulting in mergers, but also in groups and clusters containing large numbers of galaxies overall.

The majority of stars are red dwarfs: cool, low in mass, and extremely long lived.

The stars Alpha Centauri (upper left) including A and B, are part of the same trinary star system as Proxima Centauri (circled). These are the three nearest stars to Earth, and they’re located between 4.2 and 4.4 light-years away. Alpha Centauri (at left) and its slightly fainter but far more distant neighbor, Beta Centauri (at right) are easily visible in the southern skies. Proxima Centauri, the closest, is far too intrinsically faint to be seen with the unaided eye.

Our Sun, a G-class star, is more massive than 95% of stars.

This image shows the core of globular cluster Terzan 5, just 22,000 light-years away in our own Milky Way, with a wide variety of colors and masses inherent to the stars within. With millions of stars within only a few tens of light-years of one another, this dense collection of stars is still incredibly sparse, with hundreds of billions of kilometers separating the average star from its nearest neighbor. It was only through the PhD dissertation of Cecilia Payne in 1925 that we learned what stars, like the ones shown here, are made of.

Most stars are lower than ours in metallicity: the fraction of heavy elements present.

This color-coded map shows the heavy element abundances of more than 6 million stars within the Milky Way. Stars in red, orange, and yellow are all rich enough in heavy elements that they should have planets; green and cyan-coded stars should only rarely have planets, and stars coded blue or violet should have absolutely no planets at all around them. Note that the central plane of the galactic disk, extending all the way into the galactic core, has the potential for habitable, rocky planets. but that stars facing away from the galactic center (far left and right) are much lower in heavy element abundance.

Our Sun has greater enrichment than ~93% of all stars.

These charts show the estimated star-formation rate density as a function of redshift and metallicity of the stars that form. Although there are substantial uncertainties, it can be safely concluded that somewhere between only about 3% and 20% of all stars have a heavy element content that’s greater than or equal to our Sun’s, with most estimates falling between just 4-10%. However, most stars with at least ~25% of the Sun’s heavy element content possess planets.

Only half of all stars are “singlets” like our Sun; the other half exist within multi-star systems.

Although exoplanets have been found in trinary systems in recent years, most of them orbit either close to a single star or in intermediate orbits around a central binary, with the third star always being much farther away. GW Orionis is the first candidate system where planets are seen orbiting all three stars at once. About 35% of all stars are in binary systems and another 10% are in trinary systems; only about half of stars are singlets like our Sun.

We’re not typically luminous, either.

When major mergers of similarly-sized galaxies occur in the Universe, they form new stars out of the hydrogen and helium gas present within them. This can result in severely increased rates of star-formation, similar to what we observe inside the nearby galaxy Henize 2-10, located 30 million light years away. This galaxy will likely evolve, post-merger, into another disk galaxy if copious amounts of gas remains within it, or into an elliptical if all or nearly all of the gas is expelled by the current starburst. Starburst events like this were much more common earlier in cosmic history than they are today.

The overall luminosity-to-mass ratio of stars is three times our own.

Brown dwarfs, between about 0.013-0.080 solar masses, will fuse deuterium+deuterium into helium-3 or tritium, remaining at the same approximate size as Jupiter but achieving much greater masses. Red dwarfs are only slightly larger, but even the Sun-like star shown here is not shown to scale here; it would have about 7 times the diameter of a low-mass star. Stars can be up to nearly 2000 times the diameter of our Sun within this Universe.

Normal, apparently, encompasses an enormous range.

This Wolf–Rayet star is known as WR 31a, located about 30,000 light-years away in the constellation of Carina. The outer nebula is expelled hydrogen and helium, while the central star burns at over 100,000 K. In the relatively near future, many suspect that this star will explode in a core-collapse supernova much like WR 124, enriching the surrounding interstellar medium with new, heavy elements. It cannot currently be predicted which evolved, massive star in our galaxy will become the Milky Way’s next supernova.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.