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.

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 is the brightest star in the sky, but early estimates of its distance were low, as they didn’t account for the fact that Sirius is about ~20 times as intrinsically bright as our Sun.

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

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority (80%) of stars today are M-class stars, with only 1-in-800 being an O-class or B-class star massive enough for a core-collapse supernova. Our Sun is a G-class star, unremarkable but brighter than all but ~5% of stars. Only about half of all stars exist in isolation; the other half are bound up in multi-star systems.

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 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 overwhelming majority of stars were formed in the first ~4-5 billion years of our cosmic history. Only about ~15% of all stars, at maximum, have formed over the past 4.6 billion years.

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.

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 throughout cosmic time, having grown in mass and with more evolved structure at present. Younger galaxies are inherently smaller, bluer, more chaotic, richer in gas, and have lower densities of heavy elements than their modern-day counterparts, and their star-formation histories evolve over time. Most of the stars in the Universe were disproportionately formed long ago, rather than relatively recently.

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

This image shows the nearest star system to Earth: the Alpha Centauri system. The bright star toward the left of the image is both Alpha Centauri A and Alpha Centauri B, which cannot be resolved into two stars with most modern telescopes, while Proxima Centauri is very faint, and circled in red. This is presently the nearest star system to Earth; Proxima Centauri is a red dwarf, like ~75-80% of all stars, but is wildly different than a less-common star like the Sun or Alpha Centauri A.

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

This Hubble view of globular cluster Terzan 5, just 22,000 light-years away in our own Milky Way, reveals its brilliant core and stars of a wide variety of colors and masses. As gorgeous as this 2022 Hubble image is, the brightest stars within it are largest evolved giants and the highest-mass surviving stars. The majority of stars are faint and low in mass, and barely visible in an image such as this at all.

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.

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%.

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

Although planets have been found in trinary systems before in recent years, most of them orbit either close in to a single star or in intermediate orbits around a central binary, with the third star much farther away. GW Orionis is the first candidate system to have a planet 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 a star-forming region becomes so large that it extends over an entire galaxy, that galaxy becomes a starburst galaxy. Here, Henize 2-10 is shown evolving toward that state, with young stars in many locations and active stellar nurseries in numerous locations galaxy-wide. If we were to count the number of stars within the galaxy and multiply that number by the Sun’s light-to-mass ratio, we’d underestimate the total flux by about a 3-to-1 ratio.

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.

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, this star will explode in a supernova, enriching the surrounding interstellar medium with new, heavy elements. Except for the lowest-mass stars, the outer, hydrogen-rich layers of stars will get ejected back into the interstellar medium upon the cessation of nuclear fusion in the star’s core. Although Wolf-Rayet stars are rare, they’re well within the range of “normal” for a star.

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