Magnitude
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Ptolemy and the Almagest
The first attempt to compile a catalog of stars, based on the principle of their degree of luminosity, was made by the Hellenic astronomer Hipparchus of Nicaea in the 2nd century BC. Among his numerous works (unfortunately, almost all of them are lost) appeared "Star Catalog", containing a description of 850 stars classified by coordinates and luminosity. The data collected by Hipparchus, who, in addition, discovered the phenomenon of precession, was processed and received further development thanks to Claudius Ptolemy from Alexandria (Egypt) in the 2nd century. AD He created a fundamental opus "Almagest" in thirteen books. Ptolemy collected all the astronomical knowledge of that time, classified it and presented it in an accessible and understandable form. The Almagest also included the Star Catalog. It was based on observations made by Hipparchus four centuries ago. But Ptolemy’s “Star Catalog” already contained about a thousand more stars.
Ptolemy's catalog was used almost everywhere for a millennium. He divided the stars into six classes according to their luminosity: the brightest were classified as the first class, the less bright ones as the second, and so on. The sixth class includes stars that are barely visible to the naked eye. The term “luminosity of celestial bodies,” or “stellar magnitude,” is still used today to determine the measure of brilliance of celestial bodies, not only stars, but also nebulae, galaxies and other celestial phenomena.
Star brightness and visual magnitude
Looking at the starry sky, you can notice that the stars vary in their brightness or in their apparent brilliance. The brightest stars are called 1st magnitude stars; those stars that are 2.5 times fainter in brightness than 1st magnitude stars have 2nd magnitude. Those of them are classified as 3rd magnitude stars. which are 2.5 times weaker than 2nd magnitude stars, etc. The faintest stars visible to the naked eye are classified as 6th magnitude stars. It must be remembered that the name “stellar magnitude” does not indicate the size of the stars, but only their apparent brightness.
In total, there are 20 of the brightest stars in the sky, which are usually said to be stars of the first magnitude. But this does not mean that they have the same brightness. In fact, some of them are somewhat brighter than 1st magnitude, others are somewhat fainter, and only one of them is a star of exactly 1st magnitude. The same situation applies to stars of the 2nd, 3rd and subsequent magnitudes. Therefore, to more accurately indicate the brightness of a particular star, they use fractional values. So, for example, those stars that in their brightness are in the middle between stars of the 1st and 2nd magnitudes are considered to belong to the 1.5th magnitude. There are stars with magnitudes 1.6; 2.3; 3.4; 5.5, etc. Several especially bright stars are visible in the sky, which in their brilliance exceed the brilliance of stars of the 1st magnitude. For these stars, zero and negative magnitudes. So, for example, the brightest star in the northern hemisphere of the sky - Vega - has a magnitude of 0.03 (0.04) magnitude, and the brightest star - Sirius - has a magnitude of minus 1.47 (1.46) magnitude, in the southern hemisphere the brightest the star is Canopus(Canopus is located in the constellation Carina. With an apparent magnitude of minus 0.72, Canopus has the highest luminosity of any star within 700 light years of the Sun. For comparison, Sirius is only 22 times brighter than our Sun, but it is much closer to us than Canopus. For many stars among the closest neighbors of the Sun, Canopus is the brightest star in their sky.)
Magnitude in modern science
In the middle of the 19th century. English astronomer Norman Pogson
The system developed by the English astronomer made it possible to maintain the existing scale (division into six classes), but gave it maximum mathematical accuracy. First, the Polaris star was chosen as the zero point for the magnitude system; its magnitude, in accordance with the Ptolemaic system, was determined to be 2.12. Later, when it became clear that the North Star is a variable star, stars with constant characteristics were conditionally assigned to the role of the zero point. As technology and equipment improved, scientists were able to determine stellar magnitudes with greater accuracy: to tenths, and later to hundredths of units.
The relationship between apparent stellar magnitudes is expressed by Pogson's formula: m 2 -m 1 =-2.5log(E 2 /E 1) .
Number n of stars with a visual magnitude greater than L
L | n | L | n | L | n |
| 1 | 13 | 8 | 4.2*10 4 | 15 | 3.2*10 7 |
| 2 | 40 | 9 | 1.25*10 5 | 16 | 7.1*10 7 |
| 3 | 100 | 10 | 3.5*10 5 | 17 | 1.5*10 8 |
| 4 | 500 | 11 | 9*10 5 | 18 | 3*10 8 |
| 5 | 1.6*10 3 | 12 | 2.3*10 6 | 19 | 5.5*10 8 |
| 6 | 4.8*10 3 | 13 | 5.7*10 6 | 20 | 10 9 |
| 7 | 1.5*10 4 | 14 | 1.4*10 7 | 21 | 2*10 9 |
Relative and absolute magnitude
Stellar magnitude, measured using special instruments mounted in a telescope (photometers), indicates how much light from a star reaches an observer on Earth. Light travels the distance from the star to us, and, accordingly, the further away the star is, the fainter it appears. In other words, the fact that stars vary in brightness does not yet provide complete information about the star. A very bright star can have great luminosity, but be very far away and therefore have a very large magnitude. To compare the brightness of stars, regardless of their distance from the Earth, the concept was introduced "absolute magnitude". To determine the absolute magnitude, you need to know the distance to the star. The absolute magnitude M characterizes the brightness of a star at a distance of 10 parsecs from the observer. (1 parsec = 3.26 light years.). Relationship between absolute magnitude M, apparent magnitude m and distance to the star R in parsecs: M = m + 5 – 5 log R.
For relatively close stars, distant at a distance not exceeding several tens of parsecs, the distance is determined by parallax in a way that has been known for two hundred years. In this case, negligible angular displacements of stars are measured when they are observed from different points of the earth’s orbit, that is, at different times of the year. The parallaxes of even the closest stars are less than 1". The concept of parallax is associated with the name of one of the basic units in astronomy - parsec. Parsec is the distance to an imaginary star, the annual parallax of which is equal to 1".
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Do you know them all, as well as the reasons for their brightness?
I'm hungry for new knowledge. The point is to learn every day and become brighter and brighter. This is the essence of this world.
- Jay-Z
When you imagine the night sky, you most likely think of thousands of stars twinkling against the black blanket of night, something that can only be truly seen away from cities and other sources of light pollution.
But those of us who don't get to witness such a spectacle on a periodic basis are missing the fact that stars seen from urban areas with high light pollution look different than when viewed in dark conditions. Their color and relative brightness immediately set them apart from their neighboring stars, and each has its own story.
People in the northern hemisphere can probably immediately recognize Ursa Major or the letter W in Cassiopeia, while in the southern hemisphere the most famous constellation has to be the Southern Cross. But these stars are not among the ten brightest!
Milky Way next to the Southern Cross
Each star has its own life cycle, to which it is tied from the moment of birth. When any star forms, the dominant element will be hydrogen - the most abundant element in the Universe - and its fate is determined only by its mass. Stars with 8% the mass of the Sun can ignite nuclear fusion reactions in their cores, fusing helium from hydrogen, and their energy gradually moves from the inside out and pours out into the Universe. Low-mass stars are red (due to low temperatures), dim, and burn their fuel slowly—the longest-lived ones are destined to burn for trillions of years.
But the more mass a star gains, the hotter its core, and the larger the region in which nuclear fusion occurs. By the time it reaches solar mass, the star falls into class G, and its lifetime does not exceed ten billion years. Double the solar mass and you get a class A star that is bright blue and lives for less than two billion years. And the most massive stars, classes O and B, live only a few million years, after which their core runs out of hydrogen fuel. Not surprisingly, the most massive and hot stars are also the brightest. A typical class A star can be 20 times brighter than the Sun, and the most massive ones can be tens of thousands of times brighter!
But no matter how a star begins life, the hydrogen fuel in its core runs out.
And from that moment on, the star begins to burn heavier elements, expanding into a giant star, cooler, but also brighter than the original one. The giant phase is shorter than the hydrogen burning phase, but its incredible brightness makes it visible from much greater distances than the original star was visible from.
Taking all this into account, let's move on to the ten brightest stars in our sky, in increasing order of brightness.
10. Achernar. A bright blue star with seven times the mass of the Sun and 3,000 times the brightness. This is one of the fastest rotating stars known to us! It rotates so fast that its equatorial radius is 56% greater than its polar radius, and the temperature at the pole - because it is much closer to the core - is 10,000 K higher. But it is quite far from us, 139 light years away.
9. Betelgeuse. A red giant star in the Orion constellation, Betelgeuse was a bright and hot O-class star until it ran out of hydrogen and switched to helium. Despite low temperature At 3500 K, it is more than 100,000 times brighter than the Sun, which is why it is among the ten brightest, despite being 600 light years away. Over the next million years, Betelgeuse will go supernova and temporarily become the brightest star in the sky, possibly visible during the day.
8. Procyon. The star is very different from those we have considered. Procyon is a modest F-class star, just 40% larger than the Sun, and on the verge of running out of hydrogen in its core - meaning it is a subgiant in the process of evolution. It is about 7 times brighter than the Sun, but is only 11.5 light years away, so it may be brighter than all but seven stars in our sky.
7. Rigel. In Orion, Betelgeuse is not the brightest of the stars - this distinction is awarded to Rigel, a star even more distant from us. It is 860 light years away, and with a temperature of just 12,000 degrees, Rigel is not a main sequence star - it is a rare blue supergiant! It is 120,000 times brighter than the Sun, and shines so brightly not because of its distance from us, but because of its own brightness.
6. Chapel. This is a strange star because it is actually two red giants with temperatures comparable to the Sun, but each is about 78 times brighter than the Sun. At a distance of 42 light years, it is the combination of its own brightness, relatively short distance and the fact that there are two of them that allows Capella to be on our list.
5. Vega. The brightest star from the Summer-Autumn Triangle, the home of the aliens from the film “Contact”. Astronomers used it as a standard "zero magnitude" star. It is located only 25 light years from us, belongs to the stars of the main sequence, and is one of the brightest class A stars known to us, and is also quite young, only 400-500 million years old. Moreover, it is 40 times brighter than the Sun, and the fifth brightest star in the sky. And of all the stars in the northern hemisphere, Vega is second only to one star...
4. Arcturus. The orange giant, on the evolutionary scale, is somewhere between Procyon and Capella. It is the brightest star in the northern hemisphere and can be easily found by the "handle" of the Big Dipper. It is 170 times brighter than the Sun, and following its evolutionary path, it can become even brighter! It is only 37 light years away, and only three stars are brighter than it, all located in the southern hemisphere.
3. Alpha Centauri. This is a triple system in which the main member is very similar to the Sun, and is itself fainter than any star in the ten. But the Alpha Centauri system consists of the stars closest to us, so its location affects its apparent brightness - after all, it is only 4.4 light years away. Not at all like number 2 on the list.
2. Canopus. Supergiant white Canopus is 15,000 times brighter than the Sun, and is the second brightest star in the night sky, despite being 310 light-years away. It is ten times more massive than the Sun and 71 times larger - it is not surprising that it shines so brightly, but it could not reach the first place. After all, the brightest star in the sky is...
1. Sirius. It is twice as bright as Canopus, and northern hemisphere observers can often see it rising behind the constellation Orion in winter. It flickers frequently because its bright light can penetrate the lower atmosphere better than that of other stars. It's only 8.6 light-years away, but it's a class A star, twice as massive and 25 times brighter than the Sun.
It may surprise you that the top stars on the list are not the brightest or the closest stars, but rather combinations of bright enough and close enough to shine the brightest. Stars located twice as far away have four times less brightness, so Sirius shines brighter than Canopus, which shines brighter than Alpha Centauri, etc. Interestingly, class M dwarf stars, to which three out of every four stars in the Universe belong, are not on this list at all.
What we can take away from this lesson: sometimes the things that seem most striking and most obvious to us turn out to be the most unusual. Common things can be much harder to find, but that means we need to improve our observation methods!
Apparent brightness
Look at the sky at night. Most likely you will see a dozen or one and a half very bright stars (depending on the season and your location on Earth), several dozen dimmer stars and many, many very dim ones.
The brightness of stars is their oldest characteristic noticed by man. Even in ancient times, people came up with a measure for the brightness of stars - “stellar magnitude”. Although it is called “magnitude,” we are, of course, not talking about the size of the stars, but only about their brightness perceived by the eye. Some bright stars have been assigned first magnitude. For stars that looked a certain amount dimmer - the second. Stars that looked the same amount dimmer than the previous ones - the third. And so on.
Note that the brighter the star, the smaller the magnitude. Stars of the first magnitude are far from the brightest in the sky. It was necessary to enter zero magnitude and even negative ones. Fractional magnitudes are also possible. The faintest stars that the human eye can see are stars of the sixth magnitude. With binoculars you can see up to the seventh, with an amateur telescope - up to the tenth or twelfth, and the modern Hubble orbital telescope reaches up to the thirtieth.
Here are the magnitudes of our familiar stars: Sirius (-1.5), Alpha Centauri (-0.3), Betelgeuse 0.3 (on average, because variable). Everyone famous stars Ursa Major is a second magnitude star. The magnitude of Venus can reach up to (-4.5) - well, a very bright point, if you are lucky enough to see it, Jupiter - up to (-2.9).
This is how the brightness of stars has been measured for many centuries, by eye, comparing stars with standard ones. But then impartial instruments appeared, and it was discovered interesting fact. What is the apparent brightness of a star? It can be defined as the amount of light (photons) from that star that enters our eye at one time. So, it turned out that the magnitude scale is logarithmic (like all scales based on the perception of the senses). That is, the difference in brightness by one magnitude is a difference in the number of photons by two and a half times. Compare, for example, with a musical scale, it’s the same thing: an octave difference in pitch is a twofold difference in frequency.
Measuring the apparent brightness of stars in magnitudes is still used in visual observations; magnitude values are recorded in all astronomical reference books. It is convenient, for example, for quickly assessing and comparing the brightness of stars.
Radiation power
The brightness of stars that we see with our eyes depends not only on the parameters of the star itself, but also on the distance to the star. For example, the small but close Sirius looks brighter to us than the distant supergiant Betelgeuse.
To study stars, of course, you need to compare brightnesses that do not depend on distance. (They can be calculated by knowing the apparent brightness of the star, the distance to it, and an estimate of the absorption of light in a given direction.)
At first, absolute magnitude was used as such a measure - the theoretical magnitude that a star would have if placed at a standard distance of 10 parsecs (32 light years). But still, for astrophysical calculations this is an inconvenient quantity, based on subjective perception. It turned out to be much more convenient to measure not the theoretical apparent brightness, but the very real radiation power of the star. This quantity is called luminosity and is measured in luminosities of the Sun; the luminosity of the Sun is taken as one.
For reference: the luminosity of the Sun is 3.846 * 10 to the twenty-sixth power of watts.
The range of luminosities of known stars is enormous: from thousandths (and even millionths) of the sun to five to six million.
The luminosities of the stars known to us: Betelgeuse - 65,000 solar, Sirius - 25 solar, Alpha Centauri A - 1.5 solar, Alpha Centauri B - 0.5 solar, Proxima Centauri - 0.00006 solar.
But since we moved on from talking about brightness to talking about radiation power, it should be taken into account that one is not at all connected with the other unambiguously. The fact is that apparent brightness is measured only in the visible range, and stars emit far more than just this range. We know that our Sun not only shines (visible light), but also heats (infrared radiation) and causes tanning (ultraviolet radiation), and harder radiation is retained by the atmosphere. The Sun's maximum radiation falls exactly in the middle of the visible range - which is not surprising: in the process of evolution, our eyes were tuned specifically to solar radiation; For the same reason, the Sun appears completely white in vacuum. But for cooler stars, the maximum radiation is shifted to the red, or even infrared, region. There are very cool stars, such as R Doradus, that emit most of their radiation in the infrared. In hotter stars, on the contrary, the maximum radiation is shifted to the blue, violet or even ultraviolet region. Estimating the radiation power of such stars from visible radiation will be even more erroneous.
Therefore, the concept of “bolometric luminosity” of a star is used, i.e. including radiation in all ranges. Bolometric luminosity, as is clear from the above, can differ noticeably from the usual one (in the visible range). For example, the usual luminosity of Betelgeuse is 65,000 solar, and the bolometric luminosity is 100,000!
What determines the radiation power of a star?
The radiation power of a star (and therefore brightness) depends on two main parameters: temperature (the hotter it is, the more energy is emitted per unit area) and surface area (the larger it is, the more energy the star can emit at the same temperature) .
It follows from this that the brightest stars in the Universe must be blue hypergiants. This is true; such stars are called “bright blue variables.” Fortunately, there are few of them and they are all very far from us (which is extremely useful for protein life), but they include the famous “Pistol Star”, Eta Carinae and other champions of the Universe in brightness.
One thing to keep in mind is that while bright blue variables are indeed the brightest known stars (5-6 million solar luminosities), they are not the largest. Red hypergiants are much larger than blue hypergiants, but they are less luminous due to temperature.
Let's take a break from exotic hypergiants and look at main sequence stars. In principle, the processes occurring in all main sequence stars are similar (the distribution of radiation zones and convection zones in the volume of the star is different, but as long as all thermonuclear fusion occurs in the core, this does not play a special role). Therefore, the only parameter that determines the temperature of a main sequence star is mass. It's as simple as that: the heavier, the hotter. The sizes of main sequence stars are also determined by mass (for the same reason, the similarity of the structure and ongoing processes). So it turns out that the heavier, the larger and hotter, that is, the hottest stars of the main sequence are also the largest. Remember the picture with the visible colors of the stars? It illustrates this principle very well.
This means that the hottest main sequence stars are also the most powerful (brightest), and the lower their temperature, the lower their luminosity. Therefore, the main sequence on the Hertzsprung-Russell diagram has the shape of a diagonal strip from the upper left corner (the hottest stars are the brightest) to the lower right (the smallest stars are the dimmest).
There are fewer floodlights than fireflies
There is one more rule related to the brightness of stars. It was derived statistically and then explained in the theory of stellar evolution. The brighter the stars, the fewer their number.
That is, there are many more dim stars than bright ones. There are very few dazzling stars of spectral type O; there are noticeably more stars of spectral class B; there are even more stars of spectral class A, and so on. Moreover, with each spectral class the number of stars increases exponentially. So the largest stellar population in the Universe are red dwarfs - the smallest and faintest stars.
And from this it follows that our Sun is far from an “ordinary” star in terms of power, but very decent. Relatively few stars like the Sun are known, and even fewer more powerful ones.
Luminosity
For a long time, astronomers believed that the difference in the apparent brightness of stars was associated only with the distance to them: the further away the star, the less bright it should appear. But when the distances to the stars became known, astronomers discovered that sometimes more distant stars have greater apparent brightness. This means that the apparent brightness of stars depends not only on their distance, but also on the actual strength of their light, that is, on their luminosity. The luminosity of a star depends on the size of the surface of the stars and its temperature. A star's luminosity expresses its true luminous intensity compared to the luminous intensity of the Sun. For example, when they say that the luminosity of Sirius is 17, this means that the true intensity of its light is 17 times greater than the intensity of the Sun.
By determining the luminosity of stars, astronomers have found that many stars are thousands of times brighter than the Sun, for example, the luminosity of Deneb (alpha Cygnus) is 9400. Among the stars there are those that emit hundreds of thousands of times more light than the Sun. An example is the star symbolized by the letter S in the constellation Dorado. It shines 1,000,000 times brighter than the Sun. Other stars have the same or almost the same luminosity as our Sun, for example, Altair (Alpha Aquila) -8. There are stars whose luminosity is expressed in thousandths, that is, their luminous intensity is hundreds of times less than that of the Sun.
Colour, temperature and composition of stars
The stars have different color. For example, Vega and Deneb are white, Capella is yellowish, and Betelgeuse is reddish. The lower the temperature of a star, the redder it is. The temperature of white stars reaches 30,000 and even 100,000 degrees; the temperature of yellow stars is about 6000 degrees, and the temperature of red stars is 3000 degrees and below.
Stars consist of hot gaseous substances: hydrogen, helium, iron, sodium, carbon, oxygen and others.
Cluster of stars
Stars in the vast space of the Galaxy are distributed quite evenly. But some of them still accumulate in certain places. Of course, even there the distances between the stars are still very large. But due to the enormous distances, such closely located stars look like a star cluster. That's why they are called that. The most famous of the star clusters is the Pleiades in the constellation Taurus. With the naked eye, 6-7 stars can be distinguished in the Pleiades, located very close to each other. Through a telescope, more than a hundred of them are visible in a small area. This is one of the clusters in which the stars form a more or less isolated system, connected by a common movement in space. The diameter of this star cluster is about 50 light years. But even with the apparent closeness of the stars in this cluster, they are actually quite far from each other. In the same constellation, surrounding its main - the brightest - reddish star Al-debaran, there is another, more scattered star cluster - the Hyades.
Some star clusters appear as hazy, blurry spots in weak telescopes. In more powerful telescopes, these spots, especially towards the edges, break up into individual stars. Large telescopes make it possible to establish that these are particularly close star clusters, having a spherical shape. Therefore, such clusters are called globular. More than a hundred globular star clusters are now known. All of them are very far from us. Each of them consists of hundreds of thousands of stars.
The question of what the world of stars is is apparently one of the first questions that humanity has faced since the dawn of civilization. Any person contemplating the starry sky involuntarily connects the brightest stars with each other into the simplest shapes - squares, triangles, crosses, becoming the involuntary creator of his own map of the starry sky. Our ancestors followed the same path, dividing the starry sky into clearly distinguishable combinations of stars called constellations. In ancient cultures we find references to the first constellations, identified with the symbols of the gods or myths, which have come down to us in the form of poetic names - the constellation of Orion, the constellation of Canes Venatici, the constellation of Andromeda, etc. These names seemed to symbolize the ideas of our ancestors about the eternity and immutability of the universe, the constancy and immutability of the harmony of the cosmos.
How long can a star live? First, let's define it: by the lifetime of a star, we mean its ability to carry out nuclear fusion. Because the “corpse of a star” can hang for a long time even after the end of synthesis.
Typically, the less massive a star is, the longer it will live. The stars with the lowest mass are red dwarfs. They can be between 7.5 and 50 percent solar mass. Anything less massive cannot undergo nuclear fusion - and will not be a star. Current models suggest that the smallest red dwarfs can last up to 10 trillion years. Compare this with our Sun, where fusion will take approximately 10 billion years - a thousand times less. Once most of the hydrogen is fused, the theory goes, the light red dwarf will become a blue dwarf, and when the remaining hydrogen is exhausted, fusion in the core will stop and the dwarf will become white.
The oldest stars
The oldest stars appear to be those that formed immediately after the Big Bang (about 13.8 billion years ago). Astronomers can estimate the age of stars by looking at their starlight - this tells them how much of each element is in the star (eg hydrogen, helium, lithium). The oldest stars tend to be composed primarily of hydrogen and helium, with very little mass devoted to heavier elements.
The oldest star observed is SMSS J031300.36-670839.3. Its discovery was announced in February 2014. Its age is estimated at 13.6 billion years, and it is still not one of the first stars. Such stars have not yet been discovered, but they certainly could be. Red dwarfs, as we noted, live for trillions of years, but they are very difficult to detect. In any case, even if such stars exist, looking for them is like looking for a needle in a haystack.
The dimmest stars
Which stars are the dimmest? Before we answer this question, let's understand what "dim" is. The further you are from a star, the dimmer it appears, so we just need to remove distance as a factor and measure its brightness, or the total amount of energy emitted by the star in the form of photons, particles of light.
If we limit ourselves to stars that are still in the process of fusion, then the lowest luminosity is found in red dwarfs. The coolest star with the lowest luminosity currently is the red dwarf 2MASS J0523-1403. A little less light - and we will enter the kingdom of brown dwarfs, which are no longer stars.
There may also be remnants of stars: white dwarfs, neutron stars, etc. How dim can they be? White dwarfs are slightly lighter but take a long time to cool down. After a certain time, they turn into cold pieces of coal, practically not emitting light- become “black dwarfs”. White dwarfs take a very long time to cool down, so they simply don’t exist yet.
Astrophysicists do not yet know what happens to the matter of neutron stars when they cool. By observing supernovae in other galaxies, they can guess that several hundred million neutron stars must have formed in our galaxy, but only a small fraction of this number has been recorded so far. The rest must have cooled down so much that they simply became invisible.
What about black holes in deep intergalactic space with nothing in orbit? They still emit some radiation, known as Hawking radiation, but not much of it. Such lonely black holes probably glow less than the remains of stars. Do they exist? Maybe.
The brightest stars
The brightest stars also tend to be the most massive. They also tend to be Wolf-Rayet stars, which means they are hot and dump a lot of mass into strong stellar winds. The brightest stars also don't live particularly long: "live fast, die young."
The brightest star to date (and the most massive) is considered to be R136a1. Its opening was announced in 2010. It is a Wolf-Rayet star with a luminosity of approximately 8,700,000 solar and a mass 265 times greater than our home star. Once its mass was 320 solar.
R136a1 is actually part of a dense cluster of stars called R136. According to Paul Crowther, one of the discoverers, “Planets take longer to form than a star like this takes longer to live and die. Even if there were planets there, there would be no astronomers on them, because the night sky was as bright as the daytime sky."
The largest stars
Despite its enormous mass, R136a1 is not the largest star (by size). There are many larger stars, and they are all red supergiants - stars that were much smaller all their lives until they ran out of hydrogen, started fusing helium, and began to rise in temperature and expand. Our Sun will ultimately face a similar fate. The hydrogen will run out and the star will expand, turning into a red giant. To become a red supergiant, a star needs to be 10 times more massive than our Sun. The red supergiant phase is usually short, lasting only a few thousand to a billion years. This is not much by astronomical standards.
The most famous red supergiants are Alpha Antares and Betelgeuse, but they are also quite small compared to the largest. Finding the largest red supergiant is a very fruitless endeavor, because the exact sizes of such stars are very difficult to estimate for sure. The largest ones should be 1500 times wider than the Sun, maybe more.
Stars with the brightest explosions
High-energy photons are called gamma rays. They are born as a result of nuclear explosions, so some countries launch special satellites to search for gamma rays caused by nuclear tests. In July 1967, such US satellites detected an explosion of gamma rays that was not caused by a nuclear explosion. Since then, many more similar explosions have been discovered. They are usually short-lived, lasting only a few milliseconds to a few minutes. But very bright - much brighter than the brightest stars. Their source is not on Earth.
What causes gamma ray bursts? There are a lot of guesses. Today, most speculation boils down to the explosions of massive stars (supernovae or hypernovae) in the process of becoming neutron stars or black holes. Some gamma-ray bursts are caused by magnetars, a type of neutron star. Other gamma-ray bursts may be the result of two neutron stars merging into one, or a star falling into a black hole.
The Coolest Former Stars
Black holes are not stars, but the remains of stars - but they are fun to compare to stars, because such comparisons show how incredible both can be.
A black hole is what is formed when a star's gravity is strong enough to overcome all other forces and cause the star to collapse in on itself to a point of singularity. With non-zero mass but zero volume, such a point would theoretically have infinite density. However, infinities are rare in our world, so we simply don't have a good explanation for what happens at the center of a black hole.
Black holes can be extremely massive. Black holes discovered at the centers of individual galaxies can be tens of billions of solar masses. Moreover, the matter in the orbit of supermassive black holes can be very bright, brighter than all the stars in the galaxies. There may also be powerful jets near the black hole, moving almost at the speed of light.
The fastest moving stars
In 2005, Warren Brown and other astronomers at the Harvard-Smithsonian Center for Astrophysics announced the discovery of a star moving so fast that it had flown out of the Milky Way and would never return. Its official name is SDSS J090745.0+024507, but Brown called it a "rogue star."
Other fast-moving stars have also been discovered. They are known as hypervelocity stars, or ultrafast stars. As of mid-2014, 20 such stars had been discovered. Most of them seem to come from the center of the galaxy. According to one hypothesis, a pair of closely associated stars (a binary system) passed near the black hole at the center of the galaxy, one star was captured by the black hole, and the other was ejected at high speed.
There are stars that move even faster. In fact, generally speaking, the further a star is from our galaxy, the faster it is moving away from us. This is due to the expansion of the Universe, and not the movement of the star in space.
The most variable stars
The brightness of many stars fluctuates greatly when viewed from Earth. They are known as variable stars. There are many of them: in the Milky Way galaxy alone there are about 45,000 of them.
According to astrophysics professor Coel Hellier, the most variable of these stars are cataclysmic, or explosive, variable stars. Their brightness can increase by a factor of 100 during the day, decrease, increase again, and so on. Such stars are popular among amateur astronomers.
Today we have a good understanding of what happens to cataclysmic variable stars. They are binary systems in which one star is an ordinary star and the other is a white dwarf. Matter from an ordinary star falls onto an accretion disk that orbits the white dwarf. Once the mass of the disk is high enough, fusion begins, resulting in an increase in brightness. Gradually the synthesis dries up and the process begins again. Sometimes a white dwarf collapses. There are enough development options.
The most unusual stars
Some types of stars are quite unusual. They don't necessarily have extreme characteristics like luminosity or mass, they're just weird.
Like, for example, the Torna-Zytkow objects. They are named after the physicists Kip Thorne and Anna Zhitkov, who first suggested their existence. Their idea was that a neutron star could become the core of a red giant or supergiant. The idea is incredible, but... such an object was recently discovered.
Sometimes two big yellow stars circle so close to each other that, regardless of the matter that lies between them, they look like a giant cosmic peanut. Only two such systems are known.
Przybylski's Star is sometimes cited as an example of an unusual star because its starlight is different from that of any other star. Astronomers measure the intensity of each wavelength to figure out what the star is made of. This is usually not a problem, but scientists are still trying to understand the spectrum of Przybylski's star.
Based on materials from listverse.com
