The universe is there, waiting for you to discover. When we look at the universe, we are also looking back at the past. Light travels at a finite speed in the vast cosmic space. Today, many lights that reach our eyes have completed thousands of light-year journeys. On the other hand, the life span of each star is also limited. The shortest-lived stars may only live for a total of 1 to 2 million years, while other stars can live for several billion years.
Under the ideal conditions of the earth, there are about 9,000 stars that can be seen with the naked eye. The nearest star is Alpha Centauri, 4.3 light years away. The farthest one is V762 Cassiopeia, about 16,000 light years away. Most existing stars are low-mass and long-lived stars, but the brightest and easiest to see are giant stars and supergiants.
Giant stars are a class of late stars that die soon after they evolve into supernovae or planetary nebulae. Supergiants are the shortest-lived stars with a total life span of less than 10 million years.
Some stars that may have died include:
· Betelgeuse;
· Two stars on Carina Seamount;
· Spica (Alpha Virgo);
· Pegasus IK.
However, the cumulative probability of death of these stars is very low, even if the probability of death of a star is less than 1%. It is almost certain that every star we can see is still “alive”, which breaks one of the most popular legends in astronomy .
This is the Milky Way taken at the Concordia Camp in the Karakoram Mountains, Pakistan. It is often reported that many of the stars we see in the sky have burned out, but we don’t know it yet. However, this statement may be more like a legend than a true astronomical situation.
In the early 21st century, humans have successfully mapped almost all stars in the three-dimensional space of the adjacent universe. The closest star to us is not necessarily the one we can see, because the visibility of celestial bodies is determined by the combination of distance and inherent brightness.
In 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 distant stars, the light has completed the journey from the source to the observer.
When we send a light signal from the earth, it can only travel at the speed of light. A star 100 light-years away from Earth needs to wait 100 years to receive this signal. Similarly, when we look at a star 100 light years away, what we see is what it looked like 100 years ago: the light received now was emitted at that time.
The open star cluster NGC 290, taken by the Hubble Space Telescope, is about 200,000 light years away from Earth. When new stars are formed, their mass, color, brightness, and other properties all vary. The heaviest stars will be the brightest, but have the shortest lifespan; the lightest stars will have the lowest brightness, but can last for trillions of years.
Many spectacular events in space originate from typical supernovae. There are two ways to trigger a supernova, either by nuclear collapse or by thermal runaway (Type Ia). The most massive stars can reach hundreds of times the mass of the sun, and their life span is only 1 million or 2 million years. Then they will run out of fuel and die in such a huge explosion.
We can see the flat Milky Way and a few distant galaxies with the naked eye, but only a few thousand stars can be identified with the naked eye. Depending on vision and darkness, most people can see 6000 to 9000 stars if they can see the entire sky at the same time.
Alpha Centauri includes A and B (top left), which belong to the same triple star system as Proxima Centauri (red circle). These are the three closest stars to the earth, and they are 4.2 to 4.4 light years away. Alpha Centauri (left) and the slightly darker but much further neighbor Beta Centauri (right) are easily seen in the southern sky.
For ordinary astronomical observers, Cassiopeia presents a huge “W” shape, which is very easy to recognize. However, this constellation actually contains thousands of stars, and their brightness is even weaker and cannot be distinguished without astronomical equipment. Among all the stars visible to the naked eye, the farthest one is Cassiopeia V762, which is slightly lower than the second “V” in the “W” shape.
Most of the stars in the Milky Way are low-mass, low-brightness M-type stars, that is, red dwarfs in the universe. However, a considerable part of the stars we can see with the naked eye are bright and rare stars, including O, B, and A type stars, and red giant stars.
Although most of the stars in the Milky Way are low-mass and low-brightness stars, giant stars, supergiant stars, and massive stars are the easiest to see. The brightest red supergiant star, Betelgeuse (Alpha Orion) is located at the upper right of the picture and evolved from the blue supergiant at the upper left.
The Egg Nebula (also known as RAFGL 2688 or CRL 2688) taken by the Hubble Space Telescope is a “pre-planetary” nebula because its outer layer has not been heated to a sufficient temperature by the shrinking star in the center. Many of the giant stars we see today will evolve into such nebulae before they are completely abandoned in the outer layer and die in the white dwarf/planetary nebula combination.
Around Betelgeuse, the ejected material formed a nebula. The scale of Betelgeuse is shown in the red circle inside. This structure is similar to the flame emitted by a star, and the reason is that the red supergiant star is releasing its internal material into space. The extended radiation exceeds Neptune’s orbit around the sun. Betelgeuse has about 1 in 4000 chances of death.
On the left side of the Carina Nebula is the brightest star-Eta Carinae. In 2005, Haishan II was confirmed to be at least a binary star system, and some inferred that it has a larger companion. In the 19th century, Haishan II had a huge explosion, which was mistaken for a supernova event. Some people believe that this may be caused by the third companion star. Astronomers predict that Haishan II will become a supernova or polar supernova.
The Gaia Space Telescope mission deployed by the European Space Agency has mapped the distribution and three-dimensional positions of more than 1 billion stars in the Milky Way, the largest number ever. Among the stars observed by the Gaia mission, it is very likely that more than one star has died, and it can even be said that at least a few hundred stars have died. However, most of them are invisible to the naked eye, and the average distance from us may be tens of thousands of light years.
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