by Anyone interested in astronomy realizes that the stars are scattered through three-dimensional space and not, as the ancients believed, bright points attached to a celestial sphere. The night sky it has often appeared to many as a kind of roof or canopy.
Stars are generally distributed randomly in space. Their various patterns and configurations would look quite different if we could see them from elsewhere in our galaxy. Constellations as such, they are nothing more than random arrangements space. But from now on Earthwe can also get the same effect by looking out over a town or village at night and noting how the street lights and lamps in the houses can also form a ‘constellation’.
The actual intensity of each light and its distance determine how bright it appears to the viewer. A street lamp nearby, for example, can be used as an analogy Sirius, the brightest star in the sky. Indeed, the Dog Star is prominent mainly because it is only 8.6 light years from U.S. Intrinsically, it shines about 25 times brighter than our sun. But a beacon from a distant lighthouse can appear almost as bright as a street lamp despite its greater distance.
Related: The biggest and brightest stars in the galaxy
Apparent size vs. absolute size
Take the familiar summer triangle, which this week is directly overhead around 8:30 p.m. local time of day. The stars of its members – Deneb, Vegaand Altair — are at very unequal distances from us, making it a rather good example of how misleading the 2D “dome” illusion can be.
The closest of the three is Altair, which is 16.7 light-years away, followed by Vega, at 25 light-years. but Deneb is much further away, at 1,400 light years. Despite the fact that we see the last star by light that began its journey around the time the Vikings invaded Ireland, it still ranks as a first-magnitude star since it is a brilliant blue supergiant, over 100,000 times brighter than the Sun.
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But we can’t judge stellar brightness by comparing one star to another based on how they appear from our vantage point on Earth. When comparing its intrinsic brightness stars that are near and far, astronomers use a scale that measures their absolute magnitudes. By definition, the absolute luminosity of a star is the magnitude it would have when viewed from a distance of 32.6 light years.
In other words, a parsec is a unit of distance used in astronomy, equal to 3.26 light-years (3.086 × 1013 km). A parsec corresponds to the distance at which the mean radius of the Earth’s orbit subtends an angle of one arcsecond. So ten parsecs – 32.6 light years – is the “standard” distance we would measure how bright a star would appear compared to others.
If it were possible to somehow move Deneb to a distance of 10 parsecs, it would strike us at more than six times the luminosity of Aphrodite at its greatest brightness, at magnitude -6.9. On the other hand, because Vega is not much closer than 32.6 light-years, its absolute luminosity (0.6) differs little from its apparent luminosity (0.0).
Our average sun
What about our own star, The sun? At an average distance of 92.9 million miles (149.5 million km), it dazzles with a blinding apparent magnitude of -26.7. But keep in mind that compared to most other stars, the sun is nothing more than an average star. If we could observe it from a distance of 10 parsecs, it would be a faint unaided object of magnitude +4.8. If you want to get an idea of how faint this is, take a look at the Ursa Minor bowl. The faintest of the four stars that make up the bowl, Eta Ursae Minoris, is slightly fainter at magnitude +4.9.
So if we moved our sun by 10 parsecs from its current position, you’d be hard-pressed to spot it against the star background without a star atlas, and if you factor in light pollution, you wouldn’t be able to see it at all without binoculars or a small telescope.
From Pluto?
While we’re on the subject of seeing the sun from a distant location, what would it look like from the outskirts of the solar system, say from the dwarf planet, Pluto?
From there, it is often described in astronomy books as it appears as nothing more than a “very bright star”, giving the impression that it mimics Venus in the Plutonian sky. In 1960, the National Film Board of Canada produced one great documentary titled “Universe.” Speaking of Pluto, the film’s narrator, Douglas Raine (who would later provide the voice of “Hal” in the 1968 film, “2001 A Space Odyssey”) comments:
“Its surface moves in perpetual darkness and unimaginable cold, for the sun is four billion miles away, only a starry speck in the sky.”
The question here is: Is it really so?
At its average distance from the sun, Pluto is 39.4 times farther than Earth. If we use the inverse square law, the brightness of the sun there decreases by 39.4 squared, or 1,552.36. This would give us an apparent magnitude for the sun of -18.7 as seen from Pluto. Since the Full moon is magnitude -12.7, the sun from Pluto would appear as bright as 400 full moons.
Difficult “starry point” in the sky!
Catch Orion now
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If you’re an early riser, then you should definitely set your alarm for around 5am and go outside and enjoy the view of Orion, the Mighty Hunter, which at this time of the morning dominates the southeastern sky. Now, before local temperatures and weather start to get too cold and harsh, you can admire Orion and his entourage in relative comfort.
Most of the stars in this constellation are a thousand light years or more away from us. They are real galactic beacons! Alnilam, the middle star in Orion’s belt, for example, is more than 200,000 times brighter than the sun.
Two of Orion’s stars are innocent bystanders and have nothing to do with the other stars in this constellation, being much closer to us. Betelgeuse, in The hunterIts right shoulder is about 500 light-years away. It is an irregular red supergiant variable equal in luminosity to 87,000 suns. On the other shoulder is Bellatrix, about 250 light years from us. It is fortunate, of course, that these two luminaries happen to be in the same direction as the other, more distant stars in Orion, for without them we would not be able to envision a mighty hunter in the winter sky!
In the distant future, however, Orion will not appear so bright and conspicuous to us solar system is moving away from it at 12 miles per second. The peak of the sun’s motion through space is near Vega, almost across the sky from Orion.
Yet another view of the 3D sky.
Joe Rao serves as an instructor and guest lecturer in New York Hayden Planetarium (opens in new tab). He writes about astronomy for Journal of Natural History (opens in new tab)The Rural Almanac (opens in new tab) and other publications. Follow us on Twitter @Spacedotcom (opens in new tab) and up Facebook (opens in new tab).
