Astronomers classify star brightness into two categories: apparent magnitude (how bright the star looks from Earth) and absolute magnitude (how brilliant the star appears at a normal distance of 32.6 light-years, or 10 parsecs). Apparent brightness can be measured with a telescope by comparing how much brighter an object is when viewed against the night sky than it is during the day. Absolute brightness must be estimated from its position in the galaxy.
The human eye has a limited ability to see faint objects, which is why telescopes are used for viewing stars that are too dim to be seen with the unaided eye. The brightest star visible with the naked eye is Sirius, which is located in the constellation Canis Major. It can be seen just before sunrise or just after sunset. It is around 8th magnitude, which means that it is 250 times fainter than what your eye can see. A visual telescope will show you many more stars than your eye could ever see alone, while an optical telescope will show you even more distant galaxies than what your eye can see.
Absolute magnitude is based on a star's color and temperature. The brighter a star appears, the farther away it is. The farther away a star is, the faster it moves through space; thus, the faster it seems to move away from us.
The brightness of a star as viewed from Earth is determined by two factors: 1. its "absolute magnitude" (its brightness at a standard reference distance of 10 parsecs) and 2. its distance from us. Absolute magnitude is a measure of a star's luminosity; it indicates how bright a star would be if it were just inches away from you. The brighter the star, the farther away it is from us.
A star's absolute magnitude can be calculated using this formula: 0.9 × mass + 9. If we substitute values for Arcturus' mass (1.5 Msolar) and distance (25 ly), we find that it has an absolute magnitude of -14.52. A more distant but equally bright star would be Alpha Centauri, which has an absolute magnitude of -11.86.
Stars less massive than our Sun tend to burn their fuel more quickly, resulting in shorter lives. They may go supernova at the end of their life cycles and release even more energy into space. Stars more massive than our Sun usually live longer but they burn their fuel more slowly, allowing only small fractional explosions near the end of their lives. In some cases, they may completely run out of fuel and cease to burn, collapsing under their own weight to form a black hole.
Magnitude in absolute terms Why? The apparent brightness of a star is determined by two factors: the intrinsic brightness of the star and the distance between the star and the observer. Apparent brightness decreases as distance increases for a given star. So, the brighter a star is, the further away it has to be to not be seen easily from the Earth.
Intrinsic brightness The intrinsic brightness of a star is a property of the star itself and is not affected by its distance from us. It is usually expressed in units of luminosity, which is the rate at which energy is emitted by a star. A more accurate term is flux, which is the quantity of radiation emitted per unit area per second. Luminosity and flux are equal measures of solar radiation output by a star. Intrinsic brightness varies over time as stars age, with lower-mass stars becoming less bright over time.
Distance influences apparent brightness Because luminosity decreases with mass, low-mass stars appear dimmer than high-mass ones of the same color. Distance also affects how bright a star appears because telescopes can only detect photons that reach them. At greater distances, an object will seem fainter because there are fewer photons reaching the telescope from such a faraway source.
Observed magnitude vs. The perceived brightness, or apparent magnitude, is affected by the observer's position. Stars that are closer to Earth but fainter may look brighter than stars that are far away but much more brilliant. This is because their light has had more travel time across Earth's atmosphere to reach us.
Stars appear brightest to the eye on either side of the celestial equator. The reason for this is simple: The angle between the Sun and Earth is greatest at the equator, allowing most sunlight to get through our atmosphere. Polar regions experience greater atmospheric turbulence which scatters light from the stars back into space. As a result, there are proportionally fewer bright stars near the horizon when standing up compared with those same stars seen from the ground.
The sky appears darkest in the middle of the night. This is because there are no clouds in the sky to interrupt the view of distant objects like galaxies or quasars. Objects we can see only because they emit some form of radiation (such as stars or planets) are called luminous objects. Radiation travels faster than sound, so these objects always stay bright even though we cannot hear them.
Luminous objects become gradually dimmer as we move away from them.