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Starts With A Bang

Are Any Stars Visible In The Night Sky Already Dead?

Have any of the stars we can see burned themselves out completely?

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. (RICHARD POWELL / ATLAS OF THE UNIVERSE)

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. (LUCASVB / WIKIMEDIA COMMONS)

The light arriving now 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. (ESO/F. KAMPHUES)

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

The open star cluster NGC 290, 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, but will live the shortest; the lightest stars will be the least luminous, but can persist for many trillions of years. (ESA & NASA, ACKNOWLEDGEMENT: DAVIDE DE MARTIN (ESA/HUBBLE) AND EDWARD W. OLSZEWSKI (UNIVERSITY OF ARIZONA, USA))

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 or Type Ia. 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. (ISTOCK)

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

Although extended objects, like the plane 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. (BUREAU OF LAND MANAGEMENT, UNDER A CC-BY-2.0 LICENSE)

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. (WIKIMEDIA COMMONS USER SKATEBIKER)

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. (A. FUJII)

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

The overwhelming majority of stars in our galaxy are low-mass, low-luminosity M-class stars: the red dwarfs of the Universe. But a disproportionately large fraction of the stars we can see with the naked eye are bright, rare stars: O, B, and A-class stars, as well as red giant stars. (WIKIPEDIA USER KIEFF; ANNOTATIONS BY E. SIEGEL)

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. (EUROPEAN SOUTHERN OBSERVATORY)

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. 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. (NASA)

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. (ESO/P. KERVELLA)

Some compelling candidates for already dead stars are:

  1. Betelgeuse,
  2. Eta Carinae,
  3. Spica, and
  4. IK Pegasi.
The Carina Nebula, with Eta Carinae, the brightest star inside it, on the left. What appears to be a single star 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 supernova candidate today. (ESO/IDA/DANISH 1.5 M/R.GENDLER, J-E. OVALDSEN, C. THÖNE, AND C. FERON)

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. It is very likely that not only one of the stars seen by the Gaia mission has already died, but more like a few hundred at least. However, most of these are not visible to the naked eye, and are likely tens of thousands of light-years away, on average. (ESA/GAIA/DPAC, CC BY-SA 3.0 IGO)

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

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

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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