Observing Twilight into Dusk

The minutes after sunset are some of the most beautiful, as the last rays of sunlight have disappeared and the sky begins its daily transition from bright blue to dark night. Looking west after sunset in the early days of Fall, you quickly find Arcturus shining due west, at first looking almost like an airplane that is moving very slowly. But soon you realize that this is a star, the third-brightest in the heavens. It is bright enough to shine clearly through the glow of twilight. The colors of the sky are changing rapidly during twilight, and the brightest stars soon emerge. In late September, you can easily find the fifth-brightest star in the heavens, Vega, directly overhead, guiding you to its neighbors Deneb and Altair in the Summer Triangle. And looking back toward the western horizon where Arcturus is shining, you can soon find the red supergiant star Antares shining low to the left (south-west) of Arcturus.

Later this week, the young Moon arrives in the twilight sky and passes near Antares. Find a nice western horizon, get comfortable, and enjoy the evening show that is pleasant, relaxing and beautiful to see.

Image courtesy of Sky & Telescope Magazine.

Seeing the Milky Way in City Skies

Summers bring pleasant viewing and a wealth of richness in the heavens. As the Summer Triangle rises and dominates the night sky, two of my favorite Zodiac constellations, Scorpius and Sagittarius, are visible deep in the southern sky. These big, bold constellations combine with the Summer Triangle to guide the urban star-gazer to the Milky Way.

For anyone who has seen the spectacle of the Milky Way in dark skies, you know just how profound this band of light looks as we view our own galaxy from within. And I know how disappointing it is after a few days in dark skies, to return to the city and realize that the beauty of the Milky Way is missing. But don't despair, for even in urban settings, you can still see many of the wonders of the Milky Way. To do this, you need to use a telescope or binoculars. Using Scorpius and Sagittarius as your anchor on the southern horizon, and the Summer Triangle high above, point the telescope or binoculars into the sky and move slowly from the south slowly overhead. You are looking into the Milky Way, and with the magnification and light-gathering of a telescope or binoculars, you cut through the glow of the city lights and enhance the light from space. And that will be a rewarding moment under the sky. No need for a fancy star chart or an iPhone app. All you need is a sense of wonder and a comfortable chair.

Blue Star, Red Star, Yellow Star

When conducting a star party, I always point out star colors. Most of the time, people see the stars as uniformly white, but in fact upon closer inspection it's easy to see that stars have color, sometimes very dramatic color. This time of year there are several colorful bright stars that illustrate nicely the range of what you can see in the sky.

The southern sky is dominated by the distinctive shape of Scorpius, the Scorpion of the Zodiac constellations. The "heart" of this constellation is the bright red supergiant star Antares. It is in the middle of the body of the scorpion and it is one of the biggest stars we can see, so big that if places in the Solar System it would enclose Mercury, Venus, Earth and Mars. To the observer, it looks a reddish-orange color.

High above this time of year is the Summer Triangle, featuring three of the brightest stars in the sky. One of these three is Deneb in the constellation Cygnus, the "Northern Cross." Deneb appears blue to the observer and in fact is indeed a blue-white supergiant star, similar to Antares in terms of massive size, brightness and distance from the Sun.

These two beautiful stars should be enough to whet your appetite for detail when you look at the night sky. They are bright enough and easy to spot and display color quite nicely. But don't stop there. A simple pair of binoculars gets you enough resolution to see an amazing array of color in so many of the stars in the sky.

The colors of the stars is an indication of their temperature. Like the different levels of heat in flame, the colors of stars follows a similar pattern with red being cooler and blue being hotter. There are yellow stars, hotter than the red giants, and next in line are white stars, cooler than the blue stars. Details of Stellar Classification and Color Index are documented on very fine websites for those who want to learn more about star colors.

KFOG Podcast - September 10, 2010

Another fun visit with Irish Greg of the KFOG Morning Show, today featuring a discussion of constellations to see this time of year (Sagittarius, Scorpius, The Summer Triangle) and the difference between an asterism and a constellation, plus deep-space objects you can see in the Milky Way with an ordinary pair of binoculars. And the latest happenings with the Moon and Venus, and an update on a great star party and lecture this weekend. All in 7 short minutes! Click here to listen.

Dominating the Autumn Sky: Pegasus

In November, the nights grow longer and the sky changes rapidly. The Summer Triangle gradually fades into the west, and Orion and the winter constellations have not risen in the east to dominate the night sky. Instead, we see the less prominent (but no less beautiful) zodiac constellations of Capricornus, Aquarius and Pisces in the south, and directly overhead the large constellation Pegasus with the very distinctive asterism "The Great Square" in clear view. The sketch of the constellation Pegasus shown here is courtesy of the Battle Point Astronomical Association.

Pegasus is named after the Greek mythological character of the winged horse. Some constellations look a lot like their mythological namesake, but in this case I am always challenged to see the winged horse in the sky. But no matter, the shape of Pegasus is distinctive, and the Great Square makes it easy to find. This time of year Pegasus is high in the eastern sky after sunset and it moves almost directly overhead by 10 pm. I was out in my backyard a few minutes ago looking at it and to best see it I had to sit back on a chair. The Great Square is so striking because, yes, it is indeed nearly a perfect square, but more interesting is that there are virtually no other bright stars inside the square to disturb the shape. All around Pegasus, however, are a good number of bright stars, some of which form the head and feet of the winged horse, while the stars on the north side of the Great Square are actually part of Andromeda. In fact, I use Pegasus to help me find the constellation Andromeda and that points us to the Andromeda Galaxy (but that is a topic for another post).

For an excellent set of illustrations and tour of the Autumn sky, visit this webpage from the Smoky Mountain Astronomical Society.

The Summer Triangle

One of the brightest objects in the nighttime sky each summer is called the Summer Triangle. Rather than being a single constellation, the Summer Triangle is an "asterism," a grouping of stars that make an interesting pattern but are not themselves a single constellation. The Summer Triangle is, as advertised, a very distinctive triangle of three very bright stars that form a 30-60 right triangle. The three bright stars are Vega, Deneb and Altair. With three stars at magnitude 0 or 1, the Summer Triangle shines through even light-polluted city skies.

If you have darker skies, the Summer Triangle is a quick guidepost to locating some of the most interesting objects you can see with binoculars or a telescope in the Milky Way. The image to the right is taken from the website of the Great Smoky Mountain Astronomical Society. I like their web images because they present a very easy-to-follow guide to the most interesting objects in the part of the Milky Way that passes through the Summer Triangle, including double stars, nebulae, star clusters and the patterns of the three constellations that include the three bright stars of the Summer Triangle: Lyra, Aquila and Cygnus. For another description of the same objects, try this page from the Royal Observatory in Greenwich.

Use your binoculars or a telescope - even if you are in a big city - and see how much of these constellations you can identify and how many of the interesting objects you can find lurking in their midst.