March 7, 2008

In Today's One Minute Astronomer...

Magnitude: Understanding the Brightness of Astronomical Objects

Astronomers use a numerical measure called “magnitude” to describe the brightness of objects in the night sky. Here's how it works.

The Basics

• Brighter objects have a smaller numerical value of magnitude than fainter objects. So a star with magnitude 4 is brighter than a star with magnitude 5, for example.

• An object with magnitude 1.0 is 100 times brighter than an object with magnitude 6.0. So each step of 1.0 in magnitude is the fifth root of 100. That means a star of magnitude 3.0 is 2.512 times as bright as a star of magnitude 4.0, which is 2.512 times as bright as a star of magnitude 5.0, and so on. Try it yourself, if you have a calculator handy.

• With your naked eye, you can see objects down to 6th magnitude; with a pair of 7x50 binoculars you can see down to 10.5 or so; and with an 8” telescope, perhaps 13.5; using sophisticated cameras and software, the Hubble can detect objects to about 30th magnitude… about 4 billion times fainter than you can see with your eye.

• An object brighter than 0th magnitude has a negative magnitude; the brightest star, Sirius is magnitude -1.4; the full moon is magnitude -13, and the Sun is a blazing -26th magnitude.

A Deeper Look

• The “apparent” magnitude measures how bright a star appears in the sky, regardless of how bright is truly is

• “Absolute” magnitude is a measure of the true, intrinsic brightness of a star. It's defined as the apparent magnitude of an object if it was 32.616 light-years away

• While the sun has an apparent magnitude of -26, it has a modest absolute magnitude of 4.7.

• Deneb, the brightest star in Cygnus, has an absolute magnitude of -8.73, more than 250,000 times as bright as our Sun. But its apparent magnitude is only 1.25 because it's so far away, roughly 3,200 light-years from Earth.

A Bit of History

The ancient Greek astronomer Hipparchus developed the system of magnitude we use today back in 120 B.C. He used his system to catalog the brightness and position of 1,080 stars. In 1996, a European satellite named after Hipparchus created the most accurate catalog to date. It lists the precise positions of over 120,000 stars, and is available online to anyone who wants to use it.

Personal View

I've tried many times, unsuccessfully, to see the “inhumanly distant” object 3C273, a quasar in Virgo. About 1.8 billion light years from Earth, 3C273 is the brightest quasar in the sky at apparent magnitude 12.8 and absolute magnitude -26.8. I haven't had the patience to tell it apart from the background stars. The light from 3C273 that reaches us tonight left when single-cell organisms were the highest life forms on Earth.