Archive for the ‘Bright stars’ Category

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BBC article on naked eye astronomy

August 30, 2009

Milky Way over Arches NP

The BBC celebrates naked eye stargazing (hat tip to Randy).

I have mixed feelings about the piece. On one hand, it is great that such a widely-read outlet is not only featuring astronomy, but focusing on zero-equipment, zero-cost stargazing.

On the other hand, it’s a minor tragedy that their selection of objects is so out of whack with the seasons. Of the five naked-eye highlights featured in the article–Orion, Ursa Major, the Andromeda galaxy, the Pleiades, and the Milky Way–only two can be seen easily by most people right now. Those are Ursa Major (including the Big Dipper), which looks good pretty much all the time from the northern latitudes where the BBC offices are, and the Andromeda galaxy, which is just rising at sunset and well placed (up out of the near-horizon murk) by about 9:00 PM. The Pleiades don’t rise until midnight and aren’t well placed until about 2:00 AM, with Orion trailing a couple of hours behind. The Milky Way is high and bright this time of year, but tragically it is the first victim of light pollution, and if you live in or near a major metropolitan area, you can pretty much forget about seeing it unless you can travel to a dark sky site.

The list misses out on mid-summer highlights that are visible even from the city. I’ve covered several here–the stars of the Summer Triangle, especially the constellation Lyra, and Antares, the brilliant red eye of the constellation Scorpius. It’s not like “I covered them and therefore the BBC ought to have, too”. More like, “These are the best things in the sky right now, and anyone writing about introductory astronomy ought to point them out!”

The best seasonal highlight, and the one whose omission from the BBC list irks me most, is Jupiter. I haven’t written a mission about Jupiter yet, but it’s the coming thing. To the naked eye, the king of planets is the brightest star in the heavens (second only to Venus, which never strays very far from the sun), a brilliant jewel arcing across the southern sky on summer evenings. With binoculars, you can see the four Galilean moons, and even modest telescopes will reveal a few cloud bands. Last night it was overcast here but I could still see two bright patches lighting up the clouds. One was the waxing gibbous moon, just one night past first quarter, and the other was Jupiter, a small but intense spotlight off to the southeast.

So. I feel bad knocking the BBC piece. The night sky belongs to everyone and I am convinced that most people are fascinated by it and that our lives are enriched by a connection to it. Anything that gets people out there looking is therefore a good thing. I just think that arming people with a modicum of information on how to find things and what to expect is not an unreasonable expectation.

I guess that means I have to put my money where my mouth is, eh? Stand by for that Jupiter post (UPDATE: it’s up now).

Image at top from here.

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Mission 4: The Big Dipper

August 22, 2009

Mission Objectives: Constellation, Bright Stars, Multiple Stars

Equipment: Naked eye, Binoculars

Required Time: 3 minutes

Instructions: Get to a place with a clear northern horizon, look to the northwest, and find the Big Dipper. Seriously, it’s just that easy. Here, you can practice with this:

The view to the northwest right after sunset in the southern US, in Stellarium.

The view to the northwest right after sunset in the southern US, in Stellarium.

Note the little red W and N in the corners of the picture; at this time of year, the Dipper is exactly halfway between those cardinal points. If you can’t find it, make sure that it’s just after dark, see that your view isn’t blocked by clouds, trees, or mountains, and double check that you are, in fact, in the Northern Hemisphere.

The Big Dipper as a guidepost to the northern sky.

The Big Dipper as a guidepost to the northern sky.

If you can find the Dipper, you can find at least two more bright stars and have an edge on identifying their constellations. The path that is most widely known is that the two stars that make up the front end of the “pan” point unfailingly to Polaris, the North Star, around which everything else in the heavens appears to rotate. Also, you can follow the handle of the Dipper and arc to Arcturus, the brightest star in the constellation Bootes.

Like Lyra, Ursa Major has a double star treat for naked eyes and binoculars. The middle star in the handle is in fact two, Mizar and Alcor, the horse and rider. Your eyes don’t have to be particularly sharp to see that the brighter of the two, Mizar, has a dim companion. This is also a dead easy split with binoculars. A telescope working at even low magnifications of 40-50x will reveal that Mizar has another, even fainter companion, called Mizar B. Mizar was probably the first telescopic binary discovered, possibly as early as 1617, less than a decade after Galileo first aimed a telescope at the heavens. As if all of that weren’t enough, Mizar A and B are themselves both binary, although the components are too close to be separated by telescopes and can only be detected through spectroscopy.  So Mizar is a four-star system, another “double double”, all by itself.

The Big Dipper is just the rear end and oddly long tail of the constellation Ursa Major, the Great Bear. Polaris is at the end of the tail of Ursa Minor, the Little Bear. There are lots of stories about how these bears came to have such long tails–see what you can find. Because Ursa Major is so close to the celestial North Pole,  it is visible for most of the year and only dips below the horizon briefly at mid-northern latitudes. If you go far enough north, the Great Bear is visible all the time. The Greek word for bear is ‘arctos’. And so we call those far northern regions, under the eternal reign of the bear, the ‘arctic’.

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Mission 2: Eye of the Scorpion

August 18, 2009

Mission Objectives: Bright star, Constellation

Equipment: Naked eye

Required Time: 1 minute

Instructions: Go outside after dark and look south. Look for a distinctly reddish or orangey star above the southern horizon. To the right of it you will see three bright stars making a short vertical arc.

Antares in Stellarium, as it appears right after sunset in the southern part of the US.

Antares in Stellarium, as it appears right after sunset in the southern part of the US.

The red star is Antares, a red supergiant star about 600 light years away. It is an immense star, with a diameter about 800 times larger than that of the sun. If you placed Antares at the center of our solar system, it would swallow Mercury, Venus, Earth, and Mars.

Antares comparison

Antares compared to another large star, Arcturus, and the sun. R stands for 'radius'. Image from Wikipedia.

Antares is near the end of its life. Smaller stars, like our sun, eventually turn into red giants and blow off most of their mass to form planetary nebulae (so named because the near-spherical balls of gas look  like planets in telescopes, not because they actually have anything to with planets), each with a white dwarf at the core. No such “out with a whimper” fadeout for Antares–one of these days it will suffer a core collapse and blow itself apart as a Type II supernova. When this happens, the explosion will briefly outshine the entire Milky Way galaxy. Fortunately, “one of these days” could be thousands or even millions of years from now, so there’s no cause for panic.

We’re actually catching Antares pretty late in the season. The best time to see it is in late May and early June, when it is exactly opposite the sun in the sky. That means it rises at sunset, sets at dawn, and is highest in the sky in the middle of the night. I don’t know about you, but I’m not up very often at those wee hours. By August, Antares is high in the sky at sunset, or as high as it’s going to get.

Here’s the rub: Antares is pretty far south, so observers in the US and Europe need a clear southern horizon to see it. And the farther north you are, the worse your chances get. Here’s what Antares looks like just after sunset in southern England (higher and to the east you can see Altair, which I’m sure you remember from Mission 1).

Antares just after sunset from southern England.

Antares just after sunset from southern England.

Antares is also known as Alpha Scorpii, because it is the brightest star in the constellation Scorpio. The three bright stars to the right of Antares are supposed to be the scorpion’s claws, and the J-shaped hook of stars trailing off to the south and east form its body and tail, complete with a stinger on the end. To see the body and tail I have to take a short walk. From my tree-shrouded driveway all I can see are the claws and the giant red eye, as terrifying in life as it is in mythology.

The constellation Scorpio as it appears from the southern US.

The constellation Scorpio as it appears from the southern US.

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Mission 1: The Summer Triangle

August 16, 2009

Mission Objective: Bright stars

Equipment: Naked eye

Required Time: 1 minute

Instructions: Go outside after sunset and look up. If you do so not long after dark, and you face east, you’ll see three bright stars making a big triangle high in the sky. The brightest star, at the top of the triangle, will be almost directly overhead. This is Vega, in the constellation Lyra. Down and to the left of Vega is Deneb, in Cygnus (off to the northeast if you’re facing east). Down and to the right is Altair, in Aquila. Vega, Deneb, and Altair make up an informal grouping known as the Summer Triangle.

If you live under a lot of light pollution–like I do–you’ll have no problem finding the Summer Triangle because the three stars will be pretty lonely up there. Even if you’re fortunate enough to live under dark skies, the Summer Triangle is easy to spot because its members are so much brighter than anything else in the area.

The Summer Triangle in Stellarium. Click to enlarge.

The Summer Triangle in Stellarium. Click to enlarge.

To get both the Summer Triangle and the horizon in the same shot in Stellarium, I had to zoom out a lot, which introduced some fish-eye distortion. This makes it look like a cozy little polygon high up in the sky. It’s not! When Vega is high overhead, Deneb and Altair are still climbing the eastern sky, and the whole triangle takes up a huge swath of cosmic real estate.

Two more things about the picture above. You can just make out the Milky Way as a band of faint light cutting across the triangle from north to south. The whole area is packed with star clouds and other deep-sky goodies for binoculars and telescopes. And the very bright light near the southeastern horizon is Jupiter, the king of the planets, just rising to begin his stately progression across the southern sky.

But those are missions for other evenings.