Telescope Night Information

At a distance of 245,000 miles (402,000 km), the Moon, passing through the constellation Virgo tonight, is currently in a waxing gibbous phase and will reach full moon Monday. With a diameter of about 2,200 miles (~3,500 km), Earth’s moon Luna is the fifth largest moon in the solar system. A big misconception about the Moon is that we see various phases because the Earth casts a shadow on it. In reality, the phases are because half of the Moon is always illuminated (the half that happens to be facing the Sun, which changes over time as the Moon orbits the Earth), and we see varying amounts of the day/night side. Since Earth’s shadow is directly opposite the Sun, the only time the Earth could cast a shadow on the Moon is during full moon, when we see it completely illuminated! There is only one case in which the Moon does fall in Earth’s shadow - during a lunar eclipse. The Moon has two main areas of interest – the lunar highlands and maria. The highlands are the ancient, cratered surface of the Moon, whereas the maria (Latin for “seas”) are giant impact craters filled with solidified magma. The maria are much smoother because they were formed after much of the leftover debris from which the planets formed had already impacted the Moon and other bodies. Thus, there were craters where the maria are today, but they were erased when larger bodies impacted.

Found among the snaking body of the constellation Draco, the Cat’s Eye Nebula, formally known as NGC 6543, is a grand example of a planetary nebula. Despite the name, this type of object has nothing to do with planets except for a somewhat similar appearance to some of the planets in a telescope. What we’re are actually seeing is the remains of a small star that has met its demise. Though many mistakenly believe that all stars explode when they expire, the majority of stars are too small to undergo supernova explosions in their final moments. Most stars, including our own Sun in five billion years, will go through a couple of stages of swelling to many times
their original size. At this point, the outer layers of the star are very weakly held by the gravity of the collapsing core and are gently lost to space over thousands of years. During this process, the white-hot core, known as a white dwarf, is exposed to space. The copious ultraviolet light emitted by the small core is absorbed by the expanding outer layers, causing them to fluoresce like a cosmic neon sign. The white dwarf, seen at the center of the nebula, is the size of our planet, but a spoonful of it on Earth would weigh as much as an elephant!

The Great Cluster in Hercules, also known as Messier 13 or M13, is one of 150 or so known globular clusters that mostly orbit the Milky Way. This cluster is densely packed with about 300,000 stars in a sphere 150 light-years wide. By comparison, there is less than one percent of this number of stars surrounding our Sun in the sphere of the same size. At an age of more than 10 billion years, globular clusters like M13 are some of the oldest objects known. When astronomers mapped the locations of globular clusters on the sky and incorporated their distances to determine their physical locations in space, they found that the clusters had a fairly spherical distribution and that our solar system was offset from the center of that sphere. It was deduced that the distribution’s center coincided with the center of the Milky Way galaxy, which gave us one of our first estimates of how far we were located from the center of our home galaxy. On a clear, dark night, M13 is visible without a telescope, appearing as a fuzzy star in the “keystone” of Hercules. At 25,000 light-years distant, it is the farthest object associated with the Milky Way that one can see with the unaided eye.

Observe a random sampling of stars in the sky and you might be surprised just how many of them are not living their lives solo.  Many stars are in fact double stars or systems with even higher numbers of members.  Some stars, even with the most powerful telescopes, may appear singular, but the signals encoded within the light coming from them will betray the presence of another star.  This is often because we are either too far from the stars to separate them or one star's bright glare overwhelms that of its partner.

Porrima, also known as Gamma Virginia, is a great springtime double star.  It is one of brighter stars of the constellation Virgo and is located to the northwest of the constellations's brightest star, Spica.  Only 39 light-years away, it is binary star system that consists of two nearly identical stars, each with a mass and radius about 1.4 times that of the Sun.  With a temperature about 1,000K hotter than our Sun, they each emit about four times as much light as our own Sun.  The two stars are relatively close together as well.  As they orbit over a period of 169 years, they can come as close as 5AU (1 AU is the average Earth-Sun distance) of one other and move as far apart as 82AU (four times Pluto's average distance from the Sun).  For roughly a decade starting around 2010, the two stars were at or near their point of closest approach, making it possible to see them as separate stars only in the largest telescopes.  Now they have moved far enough in their orbits that modest telescopes can resolve them as separate stars.  Tonight they will appear like a small pair of headlights floating in the vast black cosmos.