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    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.

Look East in June to a supergiant that looks small to us – Deneb

How can we make the stars pop out of the sky and into our mind’s eye? That’s the perennial problem for me, for what we actually see is so much less than what is actually there that we can’t help but to either ignore or belittle the stars unintentionally. This month’s guide star, Deneb, is a prime example. It’s easy to spot using our chart as it rises in the northeast below and to the left of Vega. In terms of our bright guide star list, Deneb’s rather dim – 19th in the list of brightest stars we see with the naked eye. But that reveals much more about our point of view than about Deneb.

Deneb, plain and simple, is one of the most luminous stars in our galaxy. Vega, just above and to the right of it in the northeast, looks so much brighter – but it isn’t. It’s simply so much closer. Vega is just 25 light years away. Deneb, by the most recent calculations, is 1,425 light years from us. Put Deneb in Vega’s place and it would be visible in broad daylight!

When astronomers talk about how “luminous” a star is they don’t mean how bright it appears to us in our night sky. They mean how bright it actually is. In fact, frequently they use “luminous” to include all the radiation that comes from a star – even radiation in wavelengths that we don’t even see, such as infrared and ultraviolet. They then compare a star’s luminosity with the luminosity of the Sun – the Sun being “1.” When they examine Deneb that way they get a luminosity of 54,400 Suns – awesome! But when we look at Deneb we see a star that is just moderately bright – magnitude 1.25.


Click image for a larger version. The full "Northern Cross" asterism will appear only as the twilight dims and the night gets darker. As the month goes on it will be easier to see. This asterism is the core of the constellation Cygnus the Swan and by itself, especially as it rises, suggests a swan flying south. Around Christmas time the Northern Cross will appear to stand almost upright near the northwest horizon. You may also spot Altair, next month's guide star. (Developed from Starry Nights screen shot.)

For a printer-friendly version of this chart, click here.

When you look at Deneb, you have to use your mind’s eye to see it for what it really is, not just for what it appears to be. So what should we see when we look at Deneb?

First we should see something huge. Deneb is classed as a “supergiant.” So sit back and try to imagine a star whose diameter is 108 times that of the Sun. No, wait! First imagine how big the Earth is. Then get in your mind the fact that the Sun is 109 times the diameter of the Earth. Got that? Now try to imagine that Deneb is to our Sun what our Sun is to the Earth.

birdshotBut wait! I really do not want to talk about diameters. Those are for people who live in a flat world. Think in terms of volume, because that’s what a planet or star really is – a volume – a mass formed into a sphere. To get your mind around volume, picture the earth as a tiny bird shot just 2.5mm in diameter. Here’s one to give you the idea.

Now picture a sphere about 10.5 inches in diameter – a basketball would be close, or this glass garden globe. See the difference? When talking diameters, the Sun is 109 Earths. But when you’re talking volume, you could fit well over a million Earths inside the Sun.


Now think about the same thing in terms of Deneb. That little lead shot is our magnificent Sun. The blue globe is Deneb! That’s what you should see in your mind’s eye when you watch this month’s guide star rise in the northeast. Were Deneb our Sun, its surface would reach halfway to the orbit of Earth and needless to say, Earth would be in a hopelessly hot location.

But there’s more, of course. Size is a great starting point, but it doesn’t equate with mass. A lot of stars are bloated – that is, their mass is spread out over a large area and they have a huge surface area from which to radiate a tremendous amount of energy. That is the case with Deneb. It is believed to be about 10-15 solar masses, but its total luminosity – the total amount of energy it radiates when compared to the Sun is a whopping 54,400 times that of the Sun! Wrap your mind’s eye around that!

That’s why astronomer/author James Kaler writes that Deneb is

among the intrinsically brightest stars of its kind (that is, in its temperature or spectral class) in the Galaxy. If placed at the distance of Vega, Deneb would shine at magnitude – 7.8, 15 times more brightly than Venus at her best, be as bright as a well-developed crescent Moon, cast shadows on the ground, and easily be visible in broad daylight.

When you’re outside this June watching Deneb rise in the northeast, pause a moment and look to the northwest where brilliant Venus dominates the evening twilight. Now turn back to Deneb and imagine it 15 times as bright as Venus.
Deneb is unusual for supergiant stars for it is of spectral Class A – that means it’s your basic white star and very hot as stars go. Other very large stars, such as Betelgeuse, are in a different stage of development and quite cool and red to the eye. Deneb is believed to be just 10 million years old. That’s very young in terms of star ages. Our Sun is believed to be 5 billion years old. Deneb will never get to that ripe old age. Massive stars such as Deneb live in the fast lane, burning up their core hydrogen fuel relatively quickly.

Kaler gives this analysis:

The star is evolving and has stopped fusing hydrogen in its core. However, it’s hard to know just what is going on. It might be expanding and cooling with a dead helium core and on its way to becoming a red supergiant, or it might have advanced to the state of core helium fusion. Having begun its life as a hot class B (or even class O) star of 15 to 16 solar masses just over 10 million years ago, its fate is almost certainly to explode sometime astronomically soon as a grand supernova.

Kaler certainly knows what he’s talking about, but don’t bother to keep a “death watch” on Deneb. “Astronomically soon” means some time in the next 100 million years or so 😉

Sherlock Holmes once chided his companion Watson saying “you see, but you do not observe.” With the stars, we have to take our cue from Holmes. We have to go beyond merely seeing. And in truth, we have to go beyond merely observing. We have to take the knowledge the scientists have given us and somehow apply it to what we see, so with our mind’s eye we truly observe. Only then can we pop Deneb out of that “twinkle, twinkle little star” category and see it for what it really is.

Vital stats for Deneb (DEN-ebb), also known as Alpha Cygni:

• Brilliance: Magnitude 1.24; its luminosity is the equal of 54,400 Suns.
• Distance: 1,425 light years
• Spectral Types: A2 supergiant
• Position: 20h:41m:26s, +45°:16′:49″

Guide star reminder

Each month you’re encouraged to learn the new “guide” 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.

Deneb and the Northern Cross join several other guide stars and asterisms in the June sky. Again, if you have been reading these Posts for several months, be sure to find the stars, asterisms, and planets you found in earlier months. Early on a June evening these will include, from east to west, the following: Deneb, Vega, Arcturus, Spica, Saturn, Leo’s Rump (triangle), Mars, the Sickle, Regulus, the Beehive, and in the northwest getting near the horizon, Pollux and Castor. You may also see Capella very near the horizon, and you certainly won’t miss Venus, the a bright evening “star” dominating the sky in the west.

For more experienced observers looking to extend their knowledge of the skies this month, I highly recommend trying to track down two more asterism – the Northern Crown and the Keystone. OK, technically the Northern Crown (Corona Borealis) is a constellation. But I always apply the name to the handful of moderately bright stars that look like a half circle – a crown. As the chart below shows, these two asterisms are located on a line between Arcturus and Vega and they sort of divide that line into thirds. As with our guide stars and other asterisms, they will help you if you advance to finding other more interesting objects int he night sky with binoculars and telescope.


You'll need dark skies relatively free of light pollution to find these two asterisms. (You also should click the chart to get a larger image.) Chart was developed from "Starry Nights" screen shot.

For a printer-friendly version of this chart, click here.

The Crown itself can provide you with an interesting test of how dark your skies are since a couple hours after sunset on June night it is well up in your eastern sky. It consists of a circlet seven stars which can just fit within the field of view of wide-field binoculars – the example below shows an eight degree circle. It may be helpful to look at these stars with your binoculars, even if they don’t all fit in the same field of view at once. But to test how dark your skies are – and how transparent they are at the moment – wait until your vision is dark adapted, then see how many of these stars you can see. The numbers beside the stars are the magnitudes in decimals as given by Starry Nights software. However, I’ve followed the convention of not using a decimal point, since it might be mistaken for another faint star.  So “41” means magnitude 4.1, for example. If you are seeing all seven stars you can be happy with your skies and these light=[olluted times. In a truly dark location, however, this will be easy – but sadly such locations are rare these nights.

Read text above for explanation of how to use. Thenc lick on image to give you a larger view and luse the link below to download a printer friendly version. (Made from Starry Nights screen shot.)

For a printer-friendly version of this chart, click heret.


2 Responses

  1. Hey there.

    I’m having trouble finding the Hercules constellation, specifically the keystone. I’ve been searching for the past couple of weeks, but to no avail. It’s incredibly frustrating, since with binoculars it’s possible to form any number of “keystones”.

    I’m able to locate Vega without any problem, as well as Arcturus. However, scanning along the line from Vega immediately leaves me lost.

    I’m observing with binoculars with a 5.8 degree F.O.V. Do you know how many field of views west the keystone is from Vega? If I knew this, I’m sure it would help.

    Any other tips?

    Thanks a lot.

    • Excellent question.

      First, the next couple of weeks – in the early evening starting about two hours after sunset – will be easier because there is no moon in the sky at that time. The Keystone consist of three Magnitude 3 stars and one that is fourth magnitude. These are reasonably bright, but can easily be dimmed, if not entirely drowned out, by the Moon if it is anywhere near it. During the past week in particular the Moon would make it more difficult to see.a

      Second, you will not fit the Keystone in the field of view of your binoculars. In fact, your binoculars, if pointed at the middle of the Keystone, could show a field that fit inside it without revealing any of the four corner stars. But if you want to use your binoculars as a rough measuring device, there is about 15 degrees between Vega and the closest corner star of the Keystone – so with your binoculars that would be roughly 2.5 fields. (Or using your first held at arm’s length, about 1.5 fists.) However, unless you have very light polluted skies you want to look with your naked eye for it.

      I can well imagine that your binoculars, gathering much more light than your eyes, presented a lot of confusing stars and as you said, many keystones.. Right now the Keystone will appear to be lying on its side with the narrowest side to the south. Your binocular field should just barely include those two stars – the two that mark the narrowest side of the Keystone. But the other stars would appear alone in the field.

      Try a search without moonlight using your naked eye and I suspect you’ll have more luck. And let me know how you do! Hope this helps.

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