June 2010 Sky from the Keeble Observatory
My students often hear me say that the task of the observational astronomer is to capture a little light and ask it lots of questions. What we would like to know about a star includes such properties as its mass, luminosity, surface temperature, chemical makeup, size, etc. It turns out that knowing the actual distance to the star, while not an intrinsic property of the star itself, is also a useful datum to help us figure out the rest of those properties.
Let’s consider first finding the distance. How would you do that measurement? In fact, let’s simplify it and figure out the distance to the Sun, which is the nearest star! When asked, most of my students suggest bouncing a radar signal to measure the distance. Unfortunately, even though radars of sufficient power do exist, the Sun does not reflect any of that radiation back to us … it’s a “black body.” Rather, we use simple triangles with the Sun and Earth, and one of the planets marking the third corner. The distance to the Sun, by definition, is 1 astronomical unit – abbreviated 1AU. We use radar to measure the distance to the planet, and figure out the astronomical unit by knowing the angles and one side of the triangle. We’ll use triangles again to find the distances to nearby stars, and again use the astronomical unit as the “known” side of the triangle.
Picture an isosceles triangle (two sides are of equal length) with the unequal side spanning earth’s orbit and the opposite angle marking the position of a distant star. The apex angle of this triangle can be measured by observing the apparent shift in the star’s position relative to more distant stars. We effectively do this observation at six month intervals so the baseline is 2 AU. By definition, half of the apex angle is called the parallax angle p, which is so small that we measure it in seconds of angle. (1 degree is 60 minutes of angle; 1 minute is 60 seconds of angle; so 1 degree = 3600 seconds.) If p = 1”, we claim that the distance is d = 1/p = 1 parsec, i.e. a parallax of 1 second. The distance, from applying trigonometry to the triangle we have drawn is 206,265 AU, so this is the distance unit of 1 parsec. It turns out to be about 3.26 light years. For stars out to about 100 parsecs, we can (with great care) measure stellar distances directly. Beyond that distance the angles are so small that they cannot be measured from ground based observatories.
Next month we’ll see about determining some of the other properties of those nearby stars, now that we know their distances.Lunar phases for June: Last Quarter on the 4th, at 6:13 pm; New Moon on the 12th, at 7:15 am; First Quarter on the 19th, at 12:29 am; Full Moon on the 26th at 7:30 am.
Mercury will begin the month low on the eastern horizon at morning twilight, but will disappear into the Sun’s glare by mid-month. If you are a pre-dawn planet watcher, you’ll have to content yourself with Jupiter, which puts on a consistent show. Early in June it will be about 37 degrees above the southeast horizon. Careful observers with a small telescope may discern Uranus about one degree to the left. By the end of the month, Jupiter will slide about one degree to the left of Uranus, with both of them 50 degrees above the south-southeast horizon.
Evening planet watchers have a bit more variety. Saturn begins the month to the south in early evening twilight, about 56 degrees above the horizon – look for it to set about 11:00 pm. Mars is that bright red object next to bright blue Regulus, in Leo, about level with Saturn but to the southwest. Venus is lower to the west, about 29 degrees from the horizon and forming an inverted isosceles triangle with Castor and Pollux. Venus will move quickly against the background star field, by month’s end look for it after sunset at about the same altitude, but now closer to Regulus. Mars is west-southwest at 38 degrees elevation, and Saturn is now at 45 degrees to the southwest. (These four objects – Venus, Regulus, Mars, Saturn - will be roughly equally spaced across the sky, forming approximately a straight line.)
About two hours after sunset at mid-month, we look overhead to find the constellation Bootes at zenith. Its brightest star, Arcturus, is about 20 degrees from zenith toward the south-southwest. If we follow the line from zenith through Arcturus, the next bright star we encounter is Spica, in Virgo, at about 36 degrees from the horizon. If we turn towards the east, we find the constellation Hercules at the same height as Arcturus, i.e. about 20 degrees below zenith. Binoculars should reveal the globular cluster M13 near the upper edge of the constellation. Altair, in Aquila (the Eagle) lies about 20 degrees from the eastern horizon. To the east-northeast, Vega is the bright star 48 degrees above the horizon, in Lyra. Deneb is to the Northeast at 29 degrees. Turning to the north, Ursa Minor stretches high above Polaris … though this constellation is made up of faint stars, and is not easy to pick out. Easier is Ursa Major with its familiar Big Dipper asterism, which is majestic, high to the northwest.
Copyright 2010George Spagna