How Do I Locate Sirius in the Night Sky?

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Modern societies are rife with distractions and sources of entertainment, and if you are so inclined, you can spend a great deal of your life staring at a computer or phone screen (although you should take care to get outside for same daylight and exercise every day) and feel like you're really not missing much.

You most likely spend your evenings studying, hanging out at home with friends or family or exploring the local night life – not looking for poorly lit places so you can have a more clear view of the offerings of the sky above you.

Students and adults who are at least passively engaged with the world of astronomy may know that the brightest star in the sky (at least to Earthlings) is named Sirius, and of this group, a handful probably know that this star is nicknamed "the Dog Star" because it is in the constellation Canis Major.

As it happens, this constellation (an officially named group of stars that appear close together in the sky from Earth) is in an especially "busy" part of the sky – a regular tour de force for serious stargazers. So finding Sirius, along with the host of alluring astronomical attractions in the celestial neighborhood, is actually pretty easy.

  • To be literal, the brightest star that appears in the Earth sky is, obviously and by an absurd margin, the sun. The brightest star in the sky tonight for most Earthlings will be Sirius.

Stellar Magnitude: A "Bright" Idea

It was only natural for the earliest "official" astronomers to want to classify objects in the sky by type, and to rank them in order from brightest to dimmest. The vast majority of objects in the sky that can be seen with the unaided eye are stars.

A disproportionate fraction of the very brightest of the objects in the night sky are planets, but only five of the seven planets besides Earth itself can be seen with the naked eye: Mercury, Venus, Mars, Jupiter and Saturn.

Once humans developed the optical technology to formally measure the intensity of light falling on Earth from a distant object, then the stars could be properly put in order of how bright they look from Earth, which is called their apparent magnitude. What was still needed was to apply this technology so a scale correlating brightness with a number.

As it happens, such a system was, however imperfectly, already in place. In ancient Greece, the astronomer Hipparchus proposed a scheme that assigned the brightest stars a magnitude of 1, the stars that only the most eagle-eyed observers could spot on a clear night a 6, and the other visible stars a 2, 3, 4 or 5. This allowed for only rough differentiation, even though it was always clear that Sirius was the brightest star in the heavens.

The Old Magnitude Meets the New Magnitude

Modern scientists sought to keep the general 1-through-6 scheme for the magnitude of stars in place, but given that they now had real electromagnetic data to consider, they learned that the differences between the brightest and the dimmest visible stars was greater than these figures suggested.

What was needed was a logarithmic scale, in which numbers going up a scale increase multiplicatively (as in a certain fraction of a power of 10) rather than by the same amount with each jump. It was possible to set it up so that a first-magnitude star (1.0) would be five times brighter than a sixth-magnitude star (6.0), and that a change of 5 magnitude units implied a brightness change of 100 in the opposite direction, generally.

The Equation for Magnitude

The resulting equation for stellar magnitude is

∆M = –(5/2)log10(I/I0)

What this means is that a change in magnitude is found by taking the logarithm to the base 10 of the fraction of new intensity (I) over old intensity (I0) and then multiplying the result by – (5/2), or –2.5.

  • Another example of such a scale is the Richter scale, which measures earthquake intensity.

The apparent magnitude of Sirius is so bright that it dips into the stellar red at –1.46. Only one other star, Canopus, is "below zero." A total of 17 stand at under 1.00. When you consider what it means to be elite, being in the top 20 or so of all of the stars visible in the entire sky (and any one person can only see half of it at once) should certainly qualify.

Absolute vs. Apparent Magnitude

While it's okay to get excited and give Sirius its proper due, it is also important to realize that Sirius' advantage over the celestial competition lies chiefly in that old real-estate maxim – location, location, location. Sirius, as it happens, is only 8.6 light-years (ly) from Earth, meaning that at a distance of about (8.6 ly)(about 6 × 1012 mi/ly) = 52 trillion miles, it's actually among Earth's closest neighboring stars.

An interesting thought experiment is, "What if all the stars visible from Earth were placed the same distance away from Earth?" This would quickly reveal which of the stars in the sky had been toiling in obscurity because of sheer distance, and which had been enjoying leading roles on Earth thanks to good location instead.

In fact, scientists can and do classify objects in terms of their absolute magnitude, which is how bright something would look from a distance of 10 parsecs, or 32.6 ly. Moving Sirius back to this range would clearly take a bite out of its brilliance, and sure enough, its absolute magnitude is 1.4, fair but not truly . . . stellar. That's about as bright as the main attraction of the zodiacal constellation Leo, a star called Regulus.

Classification of Stars

One reason some stars burn more brightly than others is because they are younger and more energetic – make their behavior not unlike that of Earthlings! Also, some stars are simply born differently (e.g., more or less massive) than others.

Astronomers have divided stars into different spectral types based on temperature and assigned letters to each (for quirky historical reasons, their order is strange). In order of decreasing temperature, these are O, B, A, F, G, K and M. Within each type are subtypes given a number; for example, the friendly neighborhood star that rises reliably in the east each morning is a middle-of-the-road G2. Sirius is A1, which means "whitish and fairly hot."

  • The stars at the cooler end of the spectrum appear red, and many of the brightest stars as seen from Earth are "red giants" or "red supergiants." Examples include Arcturus, Aldebaran and Betelgeuse.
  • You can remember the order of the spectra using the saying, "Oh, Be A Fine Girl (or Guy), Kiss Me."

A Sampling of the Brightest Stars

Canopus (apparent magnitude –0.72) is never visible from much of the Northern Hemisphere. If traveling were impossible and literature nonexistent, billions of people the world over would never even know about Canopus, and that Sirius had a somewhat close rival for the honor of brightest star. Also, Canopus is 309 ly away, and its absolute magnitude is a robust –2.5.

Alpha Centauri (–0.27) might be the most famous star outside the solar system, as it's the closest at 4.3 ly. It also has allure in closely resembling the sun in spectral type (G2) and luminosity (4.4 vs. the sun's 4.2).

Rigel (0.12). This blue supergiant B8 star forms Orion's right foot (assuming Orion is facing you, and as a hunter, why wouldn't he be?). It is an extremely luminous star (absolute magnitude: –7.0). At over 800 ly away, an observer near Rigel would likely spend her entirely life oblivious to Earth's existence even if she were an astronomy professor, because the sun would not even rise to the level of a faint dot.

Betelgeuse (0.50). This M2 star, forming Orion's right shoulder has an interesting relationship with its cross-hunter counterpart, Rigel. Rigel looks slightly brighter now, but Betelgeuse is a variable star, meaning that its brightness waxes and wanes with stellar activity. It is likely for this reason that its official name is "Alpha Orionis," while Rigel gets the "Beta." Interestingly, Betelgeuse is also incredibly luminous (absolute magnitude: –7.2).

Finding Sirius

Locating Sirius is easy no matter where you are because it is close to the celestial equator, or middle of the sky. This means that people in far northern Canada can see it deep in the southern sky, and those in far southern Argentina can view it in their northern sky. Orion, for reference, is most easily seen in the southwestern night sky from November to February.

You should always have a star chart handy. You can find these online and a number of free apps are available. You can carry a mobile device with you and program it to your current date, time and location if the app doesn't do this for you. An example of a star chart website is in the Resources.

But in truth, that is a general guideline, for finding Sirius itself is quite simple. The two steps are:

  1. Find Orion, the unmistakable "hunter" constellation that looks like a giant bow tie.
  2. Follow Orion's belt to the left (Orion's right) until you hit something, which will be about the length of Orion himself from head to toe. This is Sirius.

That's really it. Even absent reference marks, Sirius is so bright that if you're at all familiar with how it looks, you could only mistake it for a planet – and except for Venus, which never wanders very close to Sirius, none of the planets display the blue-white shine of Sirius.

Sirius: Star Facts

  • Sirius' meaning in Greek is "glowing," which could be a reference not only to its brightness but to the fact that it characteristically twinkles a lot with changing atmospheric conditions. All stars do this, but it's more obvious with Sirius because of its magnitude.
  • Sirius' star constellation is named Canis Major, or "big dog." This is because the desert tribespeople who named the constellations saw the group of stars as Orion's hunting dog, or at least one of them. Nearby sits Canis Minor, or "little dog." Canis Minor has a very bright star of its own, Procyon (0.38).
  • Sirius' star location is at a right ascension of 6 hours, 45 minutes, 8.9 seconds

    and a declination of -16 degrees, 42 minutes, 58 seconds. Right ascension and declination provide the framework for astronomers to assign exact positions to stars in the sky in the same way geographers use latitude and longitude to accomplish the same thing with Earth locations. Right ascension is the "sideways" sky distance (0 to 24 hr) from a point in Aries called the vernal equinox, and declination is the distance from the celestial equator, which is the imaginary line formed by a disk extending skyward from the Earth's own equator.

References

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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