Twinkle, twinkle little star…I think I know what you are! However, there remains much to be learned about stars, and how they live and die. Until such time that we know it all, here are some amazing facts that if nothing else, should entertain you. Enjoy!
1: Stars are mostly hydrogen and helium
Hydrogen and helium were the first elements created in the Big Bang, and are the most abundant elements in the Universe today, accounting for 75% and 24% of the Universe’s mass, respectively. Similarly, these elements also form the main chemical composition of stars in roughly the same ratio of 3/4 hydrogen and 1/4 helium, with the remaining 90 naturally occurring elements all produced in stars, and representing just a small percent of a star’s overall mass. While all stars initially start out in these proportions, over time the process of stars consuming hydrogen to create energy results in helium production. That is why these proportions vary slightly, but not significantly, between stars, with our Sun, for instance, having a ratio closer to 70% hydrogen and 29% helium.
2: Our Sun is the least luminous of all “naked-eye” stars
There are around 9,096 stars that are visible to the naked eye from Earth, of which only around 2,000-2,500 can be seen without optical aid due to often less than optimal viewing conditions, as well as half the Earth being in daylight at any one time. Only a handful of these stars are of a similar brightness and size as our Sun, which is classified as a yellow dwarf. All of the rest are bigger and brighter, and in many cases, thousands of times so, especially the 400 or so stars that are brighter than 4th magnitude. Incidentally, these 400 stars form nearly the total number of stars that can be seen from urban areas. Of the 50 most luminous visible stars in the sky, Alpha Centauri is the least luminous, but even so, it is still more than one and a half times as intrinsically luminous as the sun.
3: There are more than 100 octillion stars in the Universe
A pair of binoculars allows us to see as many as half a million stars, which is still a lot less than the 100 to 200 billion stars that exist in our Milky Way galaxy. As to how many stars there are in the whole observable universe, estimates put that in the region of 100 octillion stars, based upon an estimate of 10 trillion galaxies multiplied by an average of 100 billion stars. Even then, these figures are almost certainly conservative, and until more is known about the number of galaxies, and how to estimate the number of stars in each, we will just have to keep wondering about the number of actual stars it contains.
4: Most stars occur in multiple star systems
When we look up at night, we only see single stars, but the fact is that only about 35% of all stars are loners, like our Sun. The only reason we do not see multiple star systems is because they are too far away for human vision to resolve the constituent stars in these systems. If our eyes were as big as telescopes, we would see stars that are gravitationally bound all over the sky, with some systems consisting of binary pairs that orbit other binary pairs.
5: Some of the ‘biggest’ stars are red super-giants
For convenience, astronomers measure the mass of our Sun (two nonillion kilograms) relative to that of other stars, with the unit of one solar mass therefore equal to 1. While red dwarfs are the most common type of stars found, some of the biggest stars in the universe are red giants, or more precisely, red super-giants, with Betelgeuse (Alpha Orionis) in the Orion constellation being a case in point. Although it only has about 7.7–20 times the mass of the Sun, it is at least 700-1,000 times as big, with an average diameter of around 1.643 billion km, compared to 1.392 million km for our Sun. The uncertainties arise from a number of factors, and there are no definitive size or mass values for Betelgeuse as yet.
However, it must be remembered that these stars are far along their stellar evolutionary paths, and their size is a direct result of the natural processes that occur as a star nears the end of its life. Therefore, size and mass must not be confused; size is not directly related to mass, and at current estimates, the most massive that a star can be is about 100-150 times as massive as the Sun.
6: Blue stars are the hottest
The color of a star is a function of its temperature, which in turn, is a function of its mass and composition. At the coolest end of stellar colors are red stars, which are for the most part cool red giants that have evolved off the main sequence. These stars have surface temperatures of about 2,000 or so degrees, but as stars get hotter, their colors change from red to orange, to yellow, to white, and finally to blue, which are stars that mostly fall into the Wolf-Rayet class, and have surface temperatures of anything between 25,000 and 50,000K, (24726.850C- 49726.850C ).
7: We don’t see any green stars
There would be many green stars if human vision worked differently, but the spectrum of light that is emitted by stars gets mixed up, which is why we see most stars as white, or light yellow. In fact, when our sun is viewed from outside of the Earth’s atmosphere, it is a brilliant white, and only appears to be yellow due to the way the atmosphere scatters and absorbs some of the light that enters it. The only reason why some stars, such as Zubeneschamali (Beta Librae), appear to be green is because the light from it is broken up in certain ways in some telescopes. Although most stars emit green light, the green is absorbed by other frequencies in the spectrum, which then appears as white light to human vision.
8: Stars don’t reflect radiation
Stars don’t reflect any radiation because they are black bodies, which is defined as a body that absorbs all the radiation that falls on it. However, this property of stars does not prevent them from emitting light and other forms of radiation, which is what makes them visible. In a hypothetical case where a star emits less radiation than the amount that falls on it, it would be invisible in optical light if the difference is big enough. An understanding of black body radiation lead to the development of Wien’s displacement law, which has allowed scientists to estimate the temperature of stars based upon their color.
9: Black holes do not “suck” in things
Black holes are the invisible remains of long-dead stars. Nonetheless, black holes don’t suck in surrounding matter in the manner of a vacuum cleaner, which merely create an area of lower pressure around the nozzle, with the surrounding atmospheric air rushing in to fill the low pressure area, and literally blow the dust into the hose. Black holes on the other hand, are so massive that they pull matter towards them with their immense gravitational fields, and therefore do not have to rely on pressure differences to “push” stars and gaseous clouds towards them.
10: Neutron stars are only about 30 km in diameter
Neutron stars are the remains of stars that ended their lives in titanic explosions called supernova explosions, but you probably know that already. What you may not know is that the remnant is only a few tens of kilometers in diameter, since all the molecules in the remnant have had all the empty spaces in them squashed out under the immense pressure of the explosion. A normal molecule contains more empty space than actual matter, and in a supernova explosion, all the constituent parts of the molecules in the remnant is forced into a solid mass, with no empty spaces to separate the parts. Thus, much of the original star ends up in a ball only a few kilometers across, but still possessing all of the mass of the progenitor star that was not blown off in the explosion.
Moreover, the remnant is forced to conserve its angular momentum, i.e., the original rotation of the star, which is why neutron stars spin as fast as they do. The highest known spin rate is 716 times per second, and it applies to a pulsar named PSR J1748-2446ad located about 28,000 light-years away in the constellation Sagittarius.