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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.

Events July 2013 – Hey wave – you’re on Cassini Camera!

NASA art work show how Earth might look from Saturn on July 19, 2013.

NASA art work show how Earth might look from Saturn on July 19, 2013.

Hello Saturn! Hello Earth!

NASA will help put things in perspective when you view Saturn in the southern sky on July evenings in 2013 – the space agency will show you what it’s like for Saturn to look back and view Earth!

That’s right – NASA is promising us a view from Saturn of our home planet and that’s just the sort of thing that’s fun to try to get your mind wrapped around. I’m sure it will provide us with some perspective and you can enhance your experience by first being sure to find Saturn in Earth’s evening sky – an  especially easy task this month!

Simply go out an hour after sunset and look a bit west of south – there are two bright “stars”  about a third of the way up the sky and fairly close to one another. The one on the left is Saturn – the slightly dimmer (and bluer) one is one of our guidepost stars, Spica. Wait until July 15 and you’ll see  the first quarter Moon so close to Spica it may drown it out  making the star difficult to see, except with binoculars – and the next night a slightly brighter Moon will be beneath Saturn. But any night this month the chart below will serve as a general guide – just understand the Moon is only in the vicinity for a few days near the middle of the month.

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

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

To learn when to wave  and smile for Cassini’s camera, get all the details of this historic space picture here.

Saturn is the most eye-popping object you can turn a small telescope towards  – but to the naked eye and binoculars it  looks pretty much like any bright star. But any small telescope delivering 30X or more should reveal its rings, however.

Color me red, yellow and blue!

Meanwhile, you might pick a moonless night – or one earlier in the month when the Moon is not so bright and well over in the west – and this will be a great time to test your color vision as well. The three bright guidepost stars in the chart above are very different in color. Antares is quite red, Arcturus is orange/yellow, and Spica is one of the bluest stars you’ll ever hope to see. Look at them and compare the color – don’t expect it to jump out at you. Star colors are better described as tints in my opinion – but with these three examples I think most people will see them.

For a complete description of star color and a helpful color chart, go here. The spectral type of our three stars is: Arcturus, K1; Antares, M1; and Spica B1.

Venus a constant evening star

If you have a clear and unobstructed horizon Venus should pop into view about 30-45 minutes after sunset, a fist or less above the horizon. At that time you might spot the other named stars in our chart with binoculars.  (Ckick for larger image. Chart prepared from Starry Nights Pro screenshot.)

If you have a clear and unobstructed western horizon Venus should pop into view about 30-45 minutes after sunset, a fist or less above the horizon. At that time you might spot the other named stars in our chart with binoculars. The sun will have set to the north of Venus, roughly where you see Pollux on the chart.  (Ckick for larger image. Chart prepared from Starry Nights Pro screenshot.)

Venus is a constant “evening star” during July 2013, low on the western horizon about half an hour after sunset and shining at a pretty steady magnitude  -3.9 – brilliant when compared to other planets and stars, but a bit dim for Venus.

In the course of the month as the time of sunset changes and the stars appear to slide past it and vanish, Venus seems to stay put except for a slow southward drift. It starts the month about 18 degrees north of west – but south of  where the sun sets – and concludes the month nearly due west – well, two degrees north of it as seem from my latitude of 41° 31′ North.

On July 9, 10, and 11 a fairly large crescent Moon sneaks by to the south of Venus and on the July 22, 2013  it has a real close encounter with first magnitude Regulus. They should make an interesting double star in  binoculars, barely a degree apart.

It will also be interesting to see how easy it is to pick out Regulus which will be competing with the twilight glow, as well as the much brighter Venus. I’m sure it will be easy in binoculars – but with the naked eye? Well, wait and see.

Saturn (magnitude 0.59) and Kappa Virginis (magnitude 4.15) make a similar pair of kissing cousins staying within less than a degree of one another all month. Again, binoculars may  be needed to see the star since Saturn is so relatively bright. In both cases we have separation of  4-plus magnitudes and  a degree or less.

Compare this to the classic double Mizar and Alcor – the middle star in the handle of the big Dipper. Good eyesight can separate this “horse and rider” pair, but for me it takes binoculars. With those two the separation is much smaller – about 11 minutes of arc  – roughly one fifth of a degree – but the difference in magnitude is a bit less than 2.  Bottom line – I’m sure good eyes will be able to separate the two pair mentioned – I’m not at all sure my eyes will  – though I look forward to a nice view with binoculars 😉

Look East! Drop off the slide to Spica and land on Saturn in May 2013!

If you followed “the arc” of the Big Dipper ‘s handle last month to find Arcturus, then you can do the same this month to find Spica – it’s like taking a long, cool slide from the Dipper – and if you hop off just before just before the end, this year you’ll land on Saturn!

Here’s how it looks – remember: look east  starting about an hour after Sunset.  But don’t wait too long – as the night goes on, everything will appear to rise and after a few hours this chart won’t be much help.

stuff

Start with the Big Dipper, high overhead tot he east. Following the arc of its handle, slide down to the brightest star int he east, Arcturus. Soften your slide and keep going and you’ll come to another bright star, Spica. Hopwever, if you hop off the slide just before getting to Spica, you’ll land on Saturn – which will be brighter than Spica, but not quite as bright as Arcturus.

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

While the Dipper is easy to recognize, its stars are second magnitude – bright, but not the brightest. The brightest star in this section of the sky is Arcturus at magnitude zero. (Remember, the lower the magnitude number, the brighter the object.) The next brightest star you’ll see is over to your left, low in the northeast – Vega. In fact, Vega is magnitude  zero as well and the difference between it an Arcturus is next to impossible to detect with the eye. And this year we can say the same about Saturn  – it is over near  Spica and will outshine it by just a tad and be nearly as bright as Vega and Arcturus. All of these – Arcturus, Vega, Saturn, and Spica will be brighter than any of the Dipper stars.

As these stars get higher in the sky, you will notice that Spica is a rich blue, while Saturn has a yellow tint. About an hour after sunset Vega will be the lowest at about 20-degrees above the horizon – two fists held at arms length. Saturn will be about 23 degrees above the horizon, Spica about 30 degrees, Arcturus 47 degrees and Alkaid, the star at the end of the big Dipper’s handle, about 64 degrees.

We dealt with Arcturus last month. Saturn will be in our sky most of the night and as always is a treat for the small telescope user. From a naked eye perspective,  it’s fun to remember that the name “planet” means “wanderer” in Greek, but all “wanderers” are not created equal. Mars, Venus, and Mercury move  so quickly in our night sky that you can easily mark their changes over a period of a few days -certainly a week.  Saturn is much more sluggish.

Look at the chart and you’ll see how little Saturn changes position over the course of an entire year – it moves roughly 12 degrees.  To see this in the sky , find Saturn. Hold your fist at arms length so Saturn is just below it. Just above your fist is where Saturn was last year. Put Staurn on top of your fist and just below your fist is where it will be next year. So how long will it take Saturn to get around the sky to roughly the same position? Well, 360/12 = about 30 years!  Now if you think a moment, the Moon takes about 30 days to get around our sky – and that means the Moon moves each day about 12  degrees –  the same apparent distance covered by Saturn each year.  All of which should tell you that it would be reasonable to assume Saturn is much farther away from us than the Moon – which, of course, it is.

None of this is rocket science or in any way  profound, but I find it interesting to contemplate as I look up and see Saturn. I measure that distance it will travel in the next year and in my mind’s eye I perch above the Solar System and I see a long thin pie slice reaching from me to Saturn’s distance orbit and this helps me keep things in perspective – gives me a better intuitive feel for the neighborhood in which we live.  OK – for the record Saturn is moving at about 22,000 miles an hour, Mars about 54,000 miles an hour in a much shorter orbit, and we’re whipping right along close to 67,000 miles an hour – and we don’t even feel the wind in our face! Oh – and Saturn’s actual orbital period is 29.458 years.

On to this month’s new guidepost stars!

Vega and Spica are each fascinating stars, but let’s start with Vega. Shining brightly not far above the northeastern horizon as the evening begins, Vega comes about as close to defining the word “star” as you can get. In “The Hundred Greatest Stars” James Kaler calls it “the ultimate standard star” because its magnitude is about as close to zero as you can get (.03) and its color is about as close to white as you can get. (If you’re one of those who assumed all stars are white, you’re forgiven. Individuals vary in their ability to see different colors in stars and for everyone the color differences are subtle – in fact I think of them as tints rather than colors. )

It’s hard not to be attracted to Vega when you read Leslie Peltier’s wonderful autobiography, “Starlight Nights.” Vega was central to his astronomical observing throughout his career because he began with it when he first started reading the book from which I got the idea for this web site, “The Friendly Stars” by Martha Evans Martin. Peltier wrote:

According to the descriptive text Vega, at that very hour in the month of May, would be rising in the northeastern sky. I took the open book outside, walked around to the east side of the house, glanced once more at the diagram by the light that came through the east window of the kitchen, looked up towards the northeast and there, just above the plum tree blooming by the well, was Vega. And there she had been all the springtimes of my life, circling around the pole with her five attendant stars, fairly begging for attention, and I had never seen her.

Now I knew a star! It had been incredibly simple, and all the stars to follow were equally easy.

Vega went on to be the first target of the 2-inch telescope he bought with the $18 he made by raising and picking strawberries. (This was around 1915.) And Vega became the first target for every new telescope he owned until his death in 1980. If you still don’t know a star, go out and introduce yourself to Vega early on a May evening. Even without a plum tree to look over, you can’t miss her! And once you’ve done that you’re well on your way to making the night sky your own.  (And yes, Vega is the star from which the message comes in Carl Sagan’s book/movie “Contact.”)

Vital stats for Vega, also known as Alpha Lyrae:

• Brilliance: Magnitude .03 ; a standard among stars; total radiation is that of 54 Suns.
• Distance: 25 light years
• Spectral Type: A0 Dwarf
• Position: 18h:36m:56s, +38°:47′:01″

Spica, a really bright star – honest!

Spica is truly a very bright star, but the numbers you may read for its brightness can have you pulling your hair. That’s because there are at least four common ways to express the brightness of Spica and other stars, and writers don’t always tell you which way they’re using. So let’s look at these four ways and see what they mean for Spica.

The first is the most obvious. How bright does it look to you and me from our vantage point on Earth using our eyes alone? We then assign it a brightness using the magnitude system with the lower the number, the brighter star. (For full discussion of this system, see “How bright is that star?”)

By this measure Spica is 16th on the list of brightest stars and is about as close as you can come to being exactly magnitude 1. (Officially 1.04) Though I should add here that the number really marks the midpoint of a magnitude designation – that is, any star that is in the range of magnitude .5 to magnitude 1.5 is called “magnitude 1” and so on for the other numbers on the scale.

But that scale talks about what we see. It doesn’t account for distance. Obviously if you have two 60-watt light bulbs and one is shining 6 feet away from you and the other 1,000 feet away, they are not going to look the same brightness. But if we put them both at the same distance – say 100 feet – they would look the same. So it is with stars. To compare them we pretend they all were at the same distance – in this case 10 parsecs, which is about 32.6 light years. Put our Sun at that distance and it would be magnitude 4.83. (That’s about as faint as the fainest stars we see in the Little Dipper.) We call that its absolute magnitude.

The absolute magnitude for Spica is -3.55 – not quite as bright as dazzling Venus.

Wow! That’s pretty bright compared to our Sun! Yes it is. Sun 4.83; Spica -3.55. Don’t miss the “minus” sign in front of Spica’s number! That means there’s more than eight magnitudes difference between the Sun and Spica. And that relates to the next figure you are likely to see quoted. Something that is called its luminosity. Luminosity compares the brightness of a star to the brightness of our Sun. Unfortunately, the term is often misused – or poorly defined. Thus in the Wikipedia article I just read on Spica it said that “Spica has a luminosity about 2,300 times that of the Sun.” Yes, but what does that mean? It means that if we were to put the two side by side, Spica would appear to our eyes to be 2,300 times as bright as our Sun.

That is bright! But there’s more, much more. Spica is also a very hot star – in fact one of the brightest hot stars that we see with our naked eyes. But we miss most of that brightness because most of it is being radiated in forms of energy that our eyes don’t detect. In the case of Spica, that is largely ultraviolet energy. The Wikipedia article actually listed Spica’s luminosity twice, and the second time it gave it as “13,400/1,700.”

Oh boy – now we have Spica not 2,300 times as bright as the Sun, but more than 13,000 times as bright. Now that IS bright – but is it right? Yes! So why the difference? Again, the first “luminosity” given – 2,300 times that of the Sun – is measuring only what we can see with our eyes. The second is measuring total amount of electromagnetic radiation a star radiates and is properly called the “bolometric luminosity.” And why two numbers for that last figure? 13,400/1,700? Because while Spica looks like one star to us, it is really two stars that are very close together and one is much brighter than the other. So what we see as one star is really putting out energy in the neighborhood of 15,100 times as much as our Sun.

This can get confusing, so I suggest you remember three things about Spica.

1. It defines first magnitude, having a brightness as it appears to us of 0.98 – closer than any other star to magnitude 1.

2. It is really far brighter (magnitude -3.55), but appears dim because it is far away – about 250 light years by the most recent measurements.

3. It is very hot – appearing blue to our eyes – and because it is very hot it is actually radiating a lot more energy in wavelengths we don’t see, so it is far, far brighter than our Sun.

Spica is the brightest star in the constellation Virgo, one of those constellations where you can not really connect the dots and form a picture of a virgin unless you have an over abundance of imagination. Besides, the remaining stars are relatively faint. That’s why we focus on the bright stars and sometimes those simple patterns known as “asterisms” and use them as our guides.

Vital stats for Spica, also known as Alpha Virgo:

• Brilliance: Magnitude .98 ; as close to magnitude 1 as any star gets; a close double whose combined radiation is the equal of 15,100 Suns.
• Distance: 250 light years
• Spectral Types: B1,B4 Dwarfs
• Position: 13h:25m:12s, -11°:09′:41″

Guideposts reminder

Each month you’re encouraged to learn the new “guidepost” stars rising in the east about an hour after sunset. One reason for doing this is so you can then see how they move in the following months. If you have been reading these posts for several months, you may want to relate Spica to two earlier guidepost stars with which it forms a right triangle, Arcturus and Regulus. Here’s what that triangle looks like.

Click image for larger view. (Created, with modifications, from Starry Nights Pro screen shot.)

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

Once you have identified the Right Triangle, note carefully the positions of Spica and Regulus. They pretty much mark the “ecliptic.” This is the path followed by the Sun. Also, within about 9 degrees north or south of it, you will find the planets and the Moon. That’s well illustrated in 2012 by the presence of both Saturn and Mars, very near the ecliptic, as noted on our chart.

Arcturus and Regulus are not the only guidepost stars and asterisms in the May sky. Again, if you have been reading these posts for several months, be sure to find the stars and asterisms you found in earlier months. Early on a May evening these will include, from east to west, the following: Arcturus, Spica, Saturn, Leo’s Rump (triangle), The Sickle,  Mars, Regulus, the Beehive, Procyon, Sirius, Pollux, Castor, and in the northwest near the horizon, Capella, and the Kite. Venus will be a bright evening “star” in the west, and if you look early in the month you may catch a glimpse of Sirius and Betelgeuse before they set.

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