Some of the brighter stars in the constellation Orion. The apparent magnitudes, "m", of the stars are listed. Click on image for full size (9 Kb) Original Windows to the Universe artwork by Randy Russell, using a simulated view of Orion generated by Dennis Ward.

Astronomers use the term "magnitude" to describe the brightness of an object. The magnitude scale for stars was invented by the ancient Greeks, possibly by Hipparchus around 150 B.C. The Greeks grouped the stars they could see into six brightness categories. The brightest stars were called magnitude 1 stars, while the dimmest were put in the magnitude 6 group. So, in the magnitude scale, lower numbers are associated with brighter stars.

Modern astronomers, using instruments to measure stellar brightnesses, have refined the system initially devised by the Greeks. They decided that five steps in magnitude should correspond to a brightness difference of a factor of 100. A magnitude 1 star is thus 100 times brighter than a magnitude 6 star. This means that a difference of 1 in magnitude means a factor of about 2.5 times brighter. A magnitude 3 star is 2.5 times brighter than a magnitude 4 star, while a magnitude 4 star is 2.5 times brighter than a magnitude 5 star.

Exceptionally bright objects, like the Sun, and very dim objects, such as faint stars that can only be seen with telescopes, have driven astronomers to extend the magnitude scale beyond the values of 1 through 6 used by the Greeks. We now recognize magnitudes less than one (including negative numbers) and greater than six. Sirius, the brightest star in our nighttime sky, has a magnitude of minus 1.4. The largest modern ground-based telescopes can spot stars as dim as magnitude 25 or higher. The human eye, without the aid of a telescope, can see stars as faint as 6th or 7th magnitude.

Astronomers distinguish between the apparent magnitude and the absolute magnitude of a star. "Apparent magnitude" refers to how bright a star appears to us when we view it at night from Earth. However, some stars are much further from us than others, and thus appear much dimmer simply because they are far away. Astronomers wanted a way to describe the inherent brightnesses of stars, irrespective of their proximities to us. They came up with the absolute magnitude scale. If we could place all stars a fixed distance away, and then see how bright each star looked, we would know the "absolute magnitude" of each star. We can't actually move stars around, but we can calculate how bright a star would be if placed at the agreed-upon fixed distance of ten parsecs (about 32.6 light years). Our Sun, for example, would be an inconspicuous magnitude 4.83 star at this distance. The absolute magnitude scale allows astronomers to make "apples to apples" comparisons between stars, allowing the scientists to compare the intrinsic brightnesses of stars. Astronomers often use the letter "m" to denote apparent magnitude, and "M" to signify absolute magnitude.

Whether discussing absolute or apparent magnitude, if we want to compare the magnitudes and brightnesses of two stars, we can use the following equation:

m₂ - m₁ = 2.512 (log b₁ - log b₂) = 2.512 log (b₁/b₂)

... where m₁ and m₂ are the magnitudes of star 1 and star 2, and b₁ and b₂ are the brightnesses of the two stars.

For example, Polaris, the North Pole Star, has an apparent magnitude of 1.97. Since Sirius has an apparent magnitude of minus 1.4, Sirius is [1.97 - (-1.4)] = 3.37 magnitudes brighter than Polaris. Plugging into the equation:

1.97 - (-1.4) = 3.37 = 2.512 log (b_Sirius/b_Polaris)

Solving for the brightnesses, we find that (b_Sirius/b_Polaris) = 22. In other words, Sirius appears about 22 times brighter than Polaris.

As a second example, let's say we know that a star is 5 times brighter than Mizar (one of the stars in the Big Dipper), which has an apparent magnitude of 2.23. What is the magnitude of the brighter star? We start off with he fact that (b_{bright star}/b_Mizar) = 5. We then plug into the equation:

m_Mizar - m_{bright star} = 2.23 - m_{bright star} = 2.512 log (b₁/b₂) = 2.512 log 5 = 1.76

m_{bright star} = 2.23 - 1.76 = 0.47

When you look at the nighttime sky, you can see stars with magnitudes ranging between -1.4 (Sirius, the brightest) and around 6 or 7 (the limit for the "naked eye", without binoculars or a telescope). If some of the planets are out, you might be treated to bright Venus (magnitude -4.4 at its brightest) or red Mars (which can be as brilliant as magnitude -2.8). If the Full Moon is out, you can gaze upon an object with a magnitude of -12.6. Once the Sun rises, don't look directly at it; at magnitude -26.8 you'll damage your eyes if you do!

Pluto has a magnitude around 14, so you'll need a pretty big telescope to spot it; it is much, much dimmer than the 6th magnitude stars you can barely make out with your eyes. The world's largest ground-based telescopes can detect stars in the magnitude 25 to 27 range. The Hubble Space Telescope has been able to image stars with magnitudes around 30!

This picture shows the constellation Orion. It shows the names of some of the brightest stars in Orion. It also lists the magnitudes, "m", of the stars. Which is the brightest star in Orion? Click on image for full size (9 Kb) Original Windows to the Universe artwork by Randy Russell, using a simulated view of Orion generated by Dennis Ward.

Astronomers use a special term to talk about the brightness of stars. The term is "magnitude". The magnitude scale was invented by the ancient Greeks around 150 B.C. The Greeks put the stars they could see into six groups. They put the brightest stars into group 1, and called them magnitude 1 stars. Stars that they could barely see were put into group 6. So, in the magnitude scale, bright stars have lower numbers.

A star that is one magnitude number lower than another star is about two-and-a-half times brighter. A magnitude 3 star is 2.5 times brighter than a magnitude 4 star. A magnitude 4 star is 2.5 times brighter than a magnitude 5 star.

A star that is five magnitude numbers lower than another star is exactly 100 times brighter. A magnitude 1 star is 100 times brighter than a magnitude 6 star.

Astronomers had to add some numbers to the magnitude scale since the times of the ancient Greeks. We now have lower, even negative, magnitudes for very bright objects like the Sun and Moon. We also have magnitudes higher than six for very dim stars that can be seen with telescopes.

The brightest star in the sky is Sirius. It has a magnitude of minus 1.4. The planet Mars is sometimes as bright as magnitude -2.8. Another planet, Venus, can shine as bright as magnitude -4.4. The Full Moon is a brilliant magnitude -12.6. And don't ever look at the Sun. At magnitude -26.8 the Sun's rays can damage your eyes!

Without a telescope, your eyes can just barely see magnitude 6 stars. The distant planet Pluto is magnitude 14, so you definitely need a telescope to see it. The best telescopes on Earth can spot stars with magnitudes between 25 and 27. The Hubble Space Telescope can sometimes "see" magnitude 30 stars.

There are two kinds of magnitudes for stars. One kind is apparent magnitude. The other is absolute magnitude. Apparent magnitude is how bright stars look to us in the sky from here on Earth. Remember, some stars are closer to us than other stars. A dim star that is nearby looks bright, while a very bright star that is far away looks dim. What if we could line up all of the stars the same distance away to do a fair test of their brightnesses? That is what absolute magnitude is all about.

Astronomers "pretend" to line up stars exactly 10 parsecs (about 32.6 light years) away from Earth. They then figure out how bright each star would look. They call that brightness the star's absolute magnitude. Our Sun is not an especially bright star. The Sun has an absolute magnitude of 4.83.

This picture shows the constellation Orion. It shows the names of some of the brightest stars in Orion. It also lists the magnitudes, "m", of the stars. Which is the brightest star in Orion? Click on image for full size (9 Kb) Original Windows to the Universe artwork by Randy Russell, using a simulated view of Orion generated by Dennis Ward.

Astronomers use a special term to talk about the brightness of stars. The term is "magnitude". The magnitude scale was invented by the ancient Greeks around 150 B.C. The Greeks put the stars they could see into six groups. They put the brightest stars into group 1, and called them magnitude 1 stars. Stars that they could barely see were put into group 6. So, in the magnitude scale, bright stars have lower numbers.

A star that is one magnitude number lower than another star is about two-and-a-half times brighter. A magnitude 3 star is 2.5 times brighter than a magnitude 4 star. A magnitude 4 star is 2.5 times brighter than a magnitude 5 star.

A star that is five magnitude numbers lower than another star is exactly 100 times brighter. A magnitude 1 star is 100 times brighter than a magnitude 6 star.

Astronomers had to add some numbers to the magnitude scale since the times of the ancient Greeks. We now have lower, even negative, magnitudes for very bright objects like the Sun and Moon. We also have magnitudes higher than six for very dim stars that can be seen with telescopes.

The brightest star in the sky is Sirius. It has a magnitude of minus 1.4. The planet Mars is sometimes as bright as magnitude -2.8. Another planet, Venus, can shine as bright as magnitude -4.4. The Full Moon is a brilliant magnitude -12.6. And don't ever look at the Sun. At magnitude -26.8 the Sun's rays can damage your eyes!

Without a telescope, your eyes can just barely see magnitude 6 stars. The distant planet Pluto is magnitude 14, so you definitely need a telescope to see it. The best telescopes on Earth can spot stars with magnitudes between 25 and 27. The Hubble Space Telescope can sometimes "see" magnitude 30 stars.

There are two kinds of magnitudes for stars. One kind is apparent magnitude. The other is absolute magnitude. Apparent magnitude is how bright stars look to us in the sky from here on Earth. Remember, some stars are closer to us than other stars. A dim star that is nearby looks bright, while a very bright star that is far away looks dim. What if we could line up all of the stars the same distance away to do a fair test of their brightnesses? That is what absolute magnitude is all about.

Astronomers "pretend" to line up stars exactly 10 parsecs (about 32.6 light years) away from Earth. They then figure out how bright each star would look. They call that brightness the star's absolute magnitude. Our Sun is not an especially bright star. The Sun has an absolute magnitude of 4.83.