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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

September 2013 – pursuing the not-so-false dawn – plus planets

There’s nothing false about the false dawn – in fact, it’s quite intriguing and somewhat puzzling, but very real. Here’s a cool picture of it.

Yes, I’m talking about the zodiacal light – known for hundreds, if not thousands of years as the “false dawn” because it precedes the usual predawn light. Only it isn’t always so obvious – September and October are the best time to see it in the northern hemisphere early morning sky.  (It is best seen in the early evening sky in February and March.)

Oh  – do keep in mind that the picture above was taken through the thin air and superbly dark skies above the European Southern Observatory in Chile and  benefits from the camera’s ability to do a better job of capturing faint light  than our eyes.  We won’t see it that way. But,  the picture is very useful because it gives us a good idea of the shape and size of what we are looking for when we seek this elusive glow in our skies.

If you want to catch it you have to:

  • Be out two hours before sunrise – and  give your eyes time to dark adapt. It is best seen about 80 minutes  before sunrise.
  • Be in a place relatively free of light pollution – you especially don’t want to be looking at a light dome from a city to your east. If you can see the Milky Way your skies are dark enough – if not, you need to go somewhere where you can see it.
  • Look at a time when the Moon isn’t in the morning sky – in 2013 that means the first two weeks of either September or October. 

Is it worth it – I certainly think so – but then I think September mornings are great anyways because you get to see all the bright stars of the Winter Hexagon without freezing your tail off as you do when they are in the evening sky in January. In addition we have Mars rising low in the East and Jupiter is already pretty high up and should appear near the peak of the zodiacal light – and be brighter than any star.  (Mars will be about as bright as   Castor and Pollux,  two of the bright stars  of the Winter Hexagon. Here’s a chart.

Click for larger image. Prepared from a Starry Nights Pro screen shot.

Click for larger image. Prepared from a Starry Nights Pro screen shot.

Click here for a printable, black and white version of this chart.

For you insomniacs – or folks who just love to get up early when the world is still and most of the neighbors have turned off their lights so the sky is darker, pursuing the zodiacal light is special.

What is it? It is sun light reflecting off a  huge cloud of very fine dust between the Earth and Sun on the plane of the solar system.  That’s been agreed upon for some time.  How much dust?  Well, wrap your mind around this.  Assuming that the dust particles have the same reflectivity as the surface of the moon, it would take one dust particle every five miles to reflect that much light! We’re still looking at an awful lot of empty space. Hmmm. . . there 93 million mile between the Earth and Sun – so if we had a single straight line of dust particles, we’d still have more than 18 million of them – and of course this is much more than one single line.  Now that’s awesome.

But where did all that dust come from?    J. Kelly Beatty goes over the science history in an excellent article in September’s Sky and Telescope  and notes that the current opinion is the dust cloud is a result of short period comets.  Think of a comet as a dirty snowball that melts as it nears the Sun, leaving a trail of dust. That dust stays in orbit. Short period comets are ones whose orbit takes 200 years or less because they have been captured by the gravity of the  planets. (Other comets take much longer to orbit, or simply make a single trip around the Sun.)

Why is it obvious in the morning sky in the fall and the  evening sky in the spring? Because it follows the path of the ecliptic and the ecliptic is more or less straight up and down in the morning at this time of year – and in the evening in February and March. At other times it slants at quite an angle keeping the zodiacal light lower in the sky where it gets lost in the routine dawn light.

There’s a nice little planetary show in the west this month as well, as Venus and Saturn get cozy and on September 8 Venus has a close encounter with the crescent moon right after sunset.   It’s Saturn’s turn the next night.  About a week later  Saturn and Venus should fit comfortably in the same binocular field of view for several days.

Click for a larger image. (Prepared from a Starry Nights Pro screen shot.)

Click for a larger image. (Prepared from a Starry Nights Pro screen shot.)

Look North: May is the month Polaris ( the North Star) gets two bright flankers!

Click for larger image. (Developed from Starry Nights Pro screen shot.)

Click here to download a printer-friendly version of this chart.

Is the North Star – Polaris – our brightest star? No! And it certainly won’t look that way this month as it shares the northern sky with two very bright stars. But, read on. Polaris is not nearly as dim as it looks!

If you have been learning your guidepost stars as they rise in the East, you won’t be surprised by the two bright stars which flank – and outshine – our pole star in May. To the northwest is Capella, a star we first met when it rose in the northeast in November. In May the northeast is dominated by a star that is almost Capella’s twin in brightness, Vega, a guidepost star we introduce in May. (See “Look East!” for more about Vega.) As a bonus we also have the twin guidepost stars, Castor and Pollux, making their way into the northern sky high above Capella. But let’s focus on Capella and Vega.

New star watchers frequently assume the North Star, Polaris, will be the brightest star in the sky. It isn’t even close! It is bright, but its fame comes because it’s very, very close to where the axis of the Earth points to the north celestial pole. So it serves anyone trying to find true north as a very good guide. But when it comes to brightness, it’s in the same league as the stars in the Big Dipper. Quite bright, but it can’t hold a candle to Capella and Vega. When you look at a list of the brightest stars, Vega is number 5 and Capella number 6. Polaris, our North Star, is number 48!

As simple as one, two, three!

That doesn’t mean Polaris is a slouch, though. First, in the eastern sky in May you meet Spica. (That’s on our chart for the east.) One distinction of Spica is that it’s as close to being magnitude 1 as any star gets. A distinction of Polaris is, as Spica defines magnitude 1, Polaris defines magnitude 2. (To be precise it’s magnitude 2.02.) Vega and Capella are extremely close to magnitude 0. Vega is 0.03 and Capella 0.08. Good luck on telling the difference! This month, if you look north 90 minutes after sunset, you may think Capella is a bit brighter actually – but if it appears that way it will be because it’s a bit higher in the sky and thus is not dimmed by having to fight its way through as much of our atmosphere as Vega is doing at the moment. So don’t try to split hairs. And yes, you’re right – they are NOT really as “simple as one, two, three” – on the magnitude scale they are as simple as zero, one, two – but that doesn’t sound as good! (Vega and Capella are zero; Spica is magnitude one, and Polaris, magnitude two.)

So which is really the brightest star of these four? Are you ready for this? Polaris! That’s right – if you put all four stars at the same distance, Polaris would appear to be the brightest. Remember, that the lower the magnitude number, the brighter the star. In absolute magnitude – the brightness we give to a star if they are all shining fromt he same distance  -these four stars line up this way:

  • Polaris -3.4
  • Spica -3.2
  • Capella 0.1
  • Vega 0.3

And those absolute magnitudes also reflect their order in distance from us.

  • Polaris 433 light years
  • Spica 250 light years
  • Capella 45 light years
  • Vega 25 light years

So sometimes a star is very bright because it’s – well, very bright. But sometimes it only appears to be very bright because it is very close to us. If you put our closest star into this group, our Sun – remember, it is just 8 light minutes from us – in absolute magnitude it would be by far the dimmest of this group – absolute magnitude 4.9! So while Polaris doesn’t look all that bright, it really is a very bright star! Another way to think about this is if you move our Sun out to where Polaris is, it would be about magnitude 10! You would need binoculars or a telescope to see it!

Click image for larger view of this chart. Yellow circle represents typical field of view for low power binoculars, such as 10X50.

To get an idea of the difference between Polaris and our Sun, point your binoculars towards Polaris.  You should be able to make out the “Engagement Ring” asterism – granted, a crude ring with Polaris as the diamond.  This asterism points you towards the true north celestial pole  – just avery short distance to the other side of Polaris –  and also gives you a good idea of about how far Polaris is from that pole.  Small binoculars will not show you the companion of Polaris, but to get an idea of how bright our Sun would be at the same distance, look for the star labelled 9.8 – and if you can’t see it, see if you can see the star that’s a bit brighter labelled “9.”  Don’t expect to see these instantly. Sit calmly, relax, and keep looking for at least a minute.

And here’s one more cool secret about Polaris. It has a companion that just happens to be quite dim – magnitude 9. It’s fun to see the two of them if you have a small telescope, though it’s not all that easy because Polaris is so much brighter than its companion. But if you get a chance to see Polaris and its companion in a telescope, remind yourself that the very faint companion is still a bit brighter than our Sun would look at this distance. This companion, known as Polaris B, was discovered in 1780 by William Herschel, and for many years Polaris was thought to be a binary star – that is, a system of two stars orbiting about a common center of gravity. But Polaris was holding one more surprise – it’s really a triple star.

The top image shows Polaris and its faint companion that can be seen in any decent backyard telescope. The bottom image shows the second companion, Polaris Ab, which has only been seen by using the Hubble Space Telescope.

This has been known for some time, but no one could see the third star until they turned the Hubble Space telescope on it in 2006. That’s when NASA released the first image of this third companion. The accompanying press release explained it this way:

By stretching the capabilities of NASA’s Hubble Space Telescope to the limit, astronomers have photographed the close companion of Polaris for the first time. They presented their findings  in a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C.

“The star we observed is so close to Polaris that we needed every available bit of Hubble’s resolution to see it,” said Smithsonian astronomer Nancy Evans (Harvard-Smithsonian Center for Astrophysics). The companion proved to be less than two-tenths of an arc second from Polaris — an incredibly tiny angle equivalent to the apparent diameter of a quarter located 19 miles away. At the system’s distance of 430 light years, that translates into a separation of about 2 billion miles.

“The brightness difference between the two stars made it even more difficult to resolve them,” stated Howard Bond of the Space Telescope Science Institute (STScI). Polaris is a supergiant more than two thousand times brighter than the Sun, while its companion is a main-sequence star. “With Hubble, we’ve pulled the North Star’s companion out of the shadows and into the spotlight.”

So as I said, Polaris is no slouch. It not only is a very bright star, but it also has two companions, and scientists are still studying it because it is unusual in other respects. We’ll talk about those other differences another month.

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