Have any stars visible to human eyes already died?

When we look out across the Universe, we’re also peering back in time.

In the early 21st-century, we’ve successfully mapped out practically all the stars in our neighborhood in three-dimensional space. The closest stars to us don’t always align with the stars we can see, as what’s visible is determined by a combination of distance and intrinsic brightness.

Light only travels at a finite speed across the vastness of space.

Through the vacuum of space, all light, regardless of wavelength or energy, travels at the same speed: the speed of light in a vacuum. When we observe light from a distant star, we are observing light that has already completed that journey from the source to the observer.

The light arriving now has already completed a multi-light-year journey.

When we send a light signal from Earth, it only travels at the speed of light. A star that’s located 100 light-years away will need to wait 100 years before receiving that signal. Similarly, when we look at a star 100 light-years away, we are seeing it as it was 100 years ago: when the light we’re receiving now was first emitted.

Meanwhile, every star only lives for a finite amount of time.

The open star cluster NGC 290, as imaged by Hubble. When new stars form, they form with a variety of masses, colors, luminosities, and other properties. The heaviest stars will be the most luminous and emit the greatest number of ionizing, ultraviolet photons, but will live the shortest; the lightest stars will be the least luminous but can persist for many trillions of years. Star clusters like this, themselves, typically only persist for a few hundred million years before dissociating, with many low-mass stars having the potential to get kicked to very high velocities in the process.

The shortest-lived stars may live just 1 or 2 million years total, while others survive for billions to trillions of years.

Many of the cataclysms that occur in space are typical supernovae: either core-collapse from a massive progenitor star or type Ia from an exploding white dwarf. The most massive stars of all have hundreds of times the mass of the Sun and live just 1 or 2 million years, total, before running out of fuel and dying in such a cataclysm.

Under ideal conditions on Earth, approximately 9,000 stars possess naked-eye visibilities.

Although extended objects, like the plane of the Milky Way and a few distant galaxies beyond our own, are identifiable with the naked eye, there are only a few thousand stars that can be seen and resolved with the naked eye. Depending on your eyesight and the darkness conditions, most humans can see between 6000 and 9000 stars if you could see the entire sky at once.

The closest one is Alpha Centauri: 4.3 light-years away.

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.

The farthest is V762 Cassiopeiae, some 16,000 light-years distant.

The constellation of Cassiopeia is familiar to casual skywatchers as a big “W” in the sky, but in truth the constellation contains many thousands of stars that are fainter and impossible to resolve without astronomical equipment. The farthest naked-eye star of all, V762 Cassiopeiae, can be found slightly below the second “V” in the “W” shape.

Overwhelmingly, most stars in existence are the lower-mass, longer-lived stars.

The classification system of stars by color and magnitude is very useful. By surveying our local region of the Universe, we find that only 5% of stars are greater or equal to our Sun in mass. It is thousands of times as luminous as the dimmest red dwarf star, but the most massive O-stars are millions of times as luminous as our Sun.

But the brightest ones are the easiest to see: the giants and supergiants.

Although the overwhelming majority of stars in the galaxy are low-mass and low-luminosity stars, it’s the giants, supergiants, and high-mass stars that are most easily visible. The brightest red supergiant, Betelgeuse, is shown at the upper right, having evolved from the blue supergiants at the upper left of the diagram.

Giant stars are late-stage stars, destined to die shortly in supernovae or planetary nebulae.

The Egg Nebula, as imaged here by Hubble, is a preplanetary nebula, as its outer layers have not yet been heated to sufficient temperatures by the central, contracting star to become fully ionized. Many of the giant stars visible today will evolve into a nebula like this before shedding their outer layers completely and dying in a white dwarf/planetary nebula combination. As the central star loses mass, the outermost objects in that stellar system, such as the analogue of our Oort cloud and Kuiper belt, become ejected.

The supergiants are the shortest-lived stars, with total lifetimes under 10 million years.

The nebula of expelled matter created around Betelgeuse, which, for scale, is shown in the interior red circle. This structure, resembling flames emanating from the star, forms because the behemoth is shedding its material into space. The extended emissions go beyond the equivalent of Neptune’s orbit around the Sun. Betelgeuse alone has about a 1-in-4,000 chance of having already died.

Some compelling candidates for already dead stars are:

1. Betelgeuse,
2. Eta Carinae,
3. Spica, and
4. IK Pegasi.

The Carina Nebula, shown in visible (top) and near-infrared (bottom) light, has been imaged by the Hubble Space Telescope in a series of different wavelengths, allowing these two very different views to be constructed. What appears to be a single star at the nebula’s center was identified as a binary back in 2005, and it’s led some to theorize that a third companion was responsible for triggering the supernova impostor event of the 19th century. Eta Carinae is still a compelling supernova candidate today.

But cumulative odds are slim that even one star has already died: below ~1%.

The European Space Agency’s space-based Gaia mission has mapped out the three-dimensional positions and locations of more than one billion stars in our Milky Way galaxy: the most of all-time. Looking toward the center of the Milky Way, Gaia reveals both light-blocking and luminous features that are scientifically and visually fascinating.

Every star we can see is almost certainly still alive, dispelling one of astronomy’s most popular myths.

Behind the dome of a series of European Southern Observatory telescopes, the Milky Way towers in the southern skies, flanked by the Large and Small Magellanic Clouds, at right. Although there are several thousand stars and the plane of the Milky Way all visible to human eyes, there are only four galaxies beyond our own that the typical unaided human eye can detect. We did not know they were located outside of the Milky Way until the 1920s: after Einstein’s general relativity has already superseded Newtonian gravity.

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

Ethan Siegel is on vacation this week. Please enjoy this article from the Starts With A Bang archives!