How many of you have stared at a night sky covered with thousands of sparkling lights?
Most of these are stars, which are really huge balls of burning gas. We can see the stars because they generate their own light. Stars produce extreme light and heat through nuclear reactions that take place at tens of millions of degrees Kelvin. It is the same reaction as a hydrogen bomb, except that the reaction in the stars goes on for billions of years without stopping! You have probably noticed that stars are not all the same. Some are very bright, others are very faint. They glow in different shades of color: white, yellowish, blue, orange and even red. You probably already guessed that one of the reasons that stars appear different is distance. Surprisingly, the closest star to the Sun, Proxima, in the constellation Centaurus, is not the brightest star in the sky.
Learn more about the color and magnitude of stars, main sequence stars, dead stars and stellar systems.
- Blue stars are the hottest at about 40,000K
- White stars reach 10,000K
- Yellow stars reach about 6,000K
- Orange stars reach 4,000K
The visible size of a star depends on its distance from Earth. Astronomers adjust for this to see how bright a star will be when it is a 32.6 ly away.
They call this absolute magnitude. It shows how luminous a star really is and how much energy it emits in the space compared to different stars.
The fate of a single star depends on its mass. Surprisingly, smaller stars have long and relatively stable lives, while massive stars live short and intense lives. Since small stars use their fusion of hydrogen economically, they shine evenly for billions of years.
Middle-sized stars, like our Sun, will become red giants when they exhaust their hydrogen. The central part of the star collapses and heats up until nuclear fusion of helium starts outside the core. At the same time, the outer layer of the star becomes bloated. Since the star's energy is emitted through a larger area, the surface itself becomes cooler and shines red. If the star is large enough its central parts collapse several times. Every time the central part compresses, the outer layers expand more. Such stars become super giants.
Open star clusters are groups of tens to hundreds of stars that reside same region in the Galaxy. Over time some of the stars are pulled out and become single stars, like our Sun. Others stay together as binary, triple and multiple star systems. While open star clusters consist of several hundreds of stars and have irregular shapes, globular clusters are stellar systems that contain tens thousands or even billion of stars. All known globular clusters in our Galaxy are far away from our Sun. They contain very old stars aged about 13.2 billion years, almost the same age of the Universe.
Predicted by Einstein's gravitational theory, by definition they are completely collapsed objects with a gravity that is so great that they swallow everything around them, even light!
There is no way to directly see black holes. However, we can detect their effects. If a black hole is a part of a binary star system, it can engulf the matter of the other star. The gas from the companion star spirals faster and faster, forming a disk around the black hole. Because of the extremely high speed and friction between particles, the disk heats and lights up. It also emits strong X-rays. Since there is a strong magnetic field around the black hole, some of the particles move up and down from the disk, forming powerful jets that look like cosmic projectors.
Astronomers can also calculate the mass of the invisible object by the way the companion star moves. If the mass happens to be more than three solar masses, they have discovered a black hole.