Bakersfield Night Sky – February 1, 2014
By Nick Strobel
Wow! We're already to the second month of 2014. It can't be February already, can it? But the constellations of the night sky don't lie since they're rising two hours earlier than they did a month ago. Already by 8 PM, Orion is nearly due south and most of Taurus has already passed the mid-way point (the meridian) of its nightly arc across the sky (the horns of Taurus are still east of the meridian). The stars of Orion are easy to see even in the light-polluted skies of Bakersfield. Betelgeuse at the top left corner of the central rectangle of Orion corresponds to Orion's right shoulder since the hunter is facing us.
Betelgeuse is a red supergiant star with a diameter large enough to engulf most of the asteroid belt if the star was placed in our solar system and some measurements put it even larger than Jupiter's orbit. The range of measurements of its diameter is because of several factors: differences in measurements of its distance (either 500 or 640 light years), the wavelength used to study the supergiant, and Betelgeuse's own pulsations. The transparency of Betelgeuse's gas varies with the wavelength used to observe the star. Beyond the visible (or optical) band of light that our eyes can see are the longer wavelengths of light called infrared. That form of light is what we can feel as heat. Observations at the longer wavelengths of infrared give the larger diameter for Betelgeuse while the observations at the infrared wavelengths just slightly longer than the red color of the visible rainbow show a smaller star. A further complicating factor is that Betelgeuse is embedded in many shells of dust and gas from numerous ejections of material in its old age that extend out to 20,000 times Earth's orbit size. Betelgeuse has lost an amount of gas equivalent to the mass of the entire Sun in these burps. Eventually Betelgeuse will go supernova becoming as bright a nearly full Moon, so we'll be able to see it in broad daylight. We're far enough away from Betelgeuse that we won't be harmed from the blast whenever it occurs. No, no one knows when it will explode but a pulse of neutrinos detected by one of the few neutrino observatories now online will be the first sign that the core collapse that triggers the supernova has happened. Astronomer Jim Kaler has more details about Betelgeuse on his Stars and Constellations site at http://stars.astro.illinois.edu/sow/sow.html .
Back to Orion. At the top right corner is the star Bellatrix, a star so much hotter than the Sun that it is blue-white in color. It is about 240 light years away and it may be beginning the final stages of its life. Bellatrix corresponds to Orion's left shoulder. At the bottom right corner (Orion's left knee or thigh) is the brilliant Rigel, also blue-white hot. It is also a supergiant but it is just beginning to swell outward so it is now "only" the diameter of Mercury's orbit. Rigel is 860 light years away and it has a pair of smaller hot stars that orbits the supergiant at 60 times Pluto's orbit around the Sun plus a small orange star cooler than the Sun that orbits all three in an orbit at least 290 times Pluto's orbit every 250,000 years. At the bottom left corner of Orion is Saiph, a blue supergiant hotter than Rigel and Bellatrix and not as far along in its life stage as the other three. Saiph is 720 light years away. At Orion's middle is his famous belt stars: Alnitak, Alnilam, and Mintaka, in order from left to right on the sky. I wrote about the belt stars last April in this column. Coming down from the belt is Orion's sword with a fuzzy patch in the middle of the sword that is the great Orion Nebula, a giant star formation factory about 1350 light years away from us. Several hundred stars and planetary systems are forming inside of it.
Follow the line of Orion's belt upward to the right of Orion and you'll come to the bright orange giant star Aldebaran at the eye of Taurus the bull. The beautiful cluster Pleiades is at Taurus' shoulder. The Pleiades are named after the seven daughters of the titan Atlas and sea-nymph Pleione of Greek mythology but only six stars are clearly visible to the naked eye. Binoculars and a good telescope will show several dozen to over a thousand stars bound together gravitationally into a group about 43 light years across and about 440 light years away from us. The V-shaped nose of Taurus is formed by the closest open cluster to us, the Hyades, that is only 153 light years away from us. Although, Aldebaran is at the end of the left part of the V, it is not part of the Hyades because Aldebaran is less than half the distance from us than the Hyades.
To the lower right of Orion is a faint string of stars in a constellation that winds southward called Eridanus, that is the Latin name of the Po River in Italy. Right below (south of) Orion is Lepus, the rabbit being chased by Orion and his hunting dogs that are to the left of Orion. Follow the line of Orion's belt downward to the left and you'll come to the brightest star in all of the night sky, Sirius (the "Dog Star"), at the nose or neck (depending on how you connect the dots) of Canis Major, one of the Orion's hunting dogs. Sirius is so bright because it is very near to us at just 8.6 light years away and because it is white hot with an energy output 26 times that of the Sun. Sirius is the closest star that we can see from Bakersfield without binoculars or a telescope. People closer to the equator and in the southern hemisphere can see the closest system of Alpha Centauri. Sirius has a white dwarf companion (Sirius B) about half as massive as Sirius, so the two stars orbit a common point every 50 years that is twice as close to bright Sirius than it is to the white dwarf. A white dwarf is a cooling, dead core remnant of a star and it is very compressed. Sirius B is about the mass of the Sun but has a diameter smaller than the Earth which means its density is about 1.7 metric tons per cubic centimeter. Our Sun will eventually end up as a white dwarf too but it won't be as compressed as Sirius B because the Sun will have less gravity compression. J K Rowling named one of the important characters in the Harry Potter series after Sirius (and another after Orion's Bellatrix). Near Sirius is another open cluster called M 41 that is visible to the naked eye if you are away from the city. Smaller than the Pleiades and the Hyades, M41 has only about 100 stars and it is much farther away at a distance of 2300 light years away from us.
Immediately left of Orion is a constellation made of faint stars called Monoceros, the unicorn. Above that is smaller hunting dog, Canis Minor, with bright Procyon. Procyon is slightly hotter than the Sun and it is only 11.4 light years away from us. Like Sirius, Procyon has a white dwarf companion and they orbit each other with a period of 40.8 years at an average separation of almost Uranus's distance from the Sun. To Orion's upper left are Pollux and Castor, the Gemini twins. The two brightest stars in Gemini are at the heads of the twins and named after them. Pollux is an orange giant star slightly cooler than Sun but about 10 times the diameter of the Sun and it is 34 light years away. Pollux is also the brightest star with an exoplanet orbiting it. Castor is a white-hot quadruple star system 51 light years away from us. How I keep Pollux and Castor straight in my mind is that Pollux is closer to Procycon while Castor is closer to Capella of Auriga (so the two "P"-named stars are close together and the two "C"-named stars are close together). Completing our tour around Orion is Auriga, the charioteer at the zenith straight overhead at 8 PM. Auriga has the very bright star Capella. Capella is also a quadruple star system and it is 43 light years away.
The band of the Milky Way goes to the left of Canis Major, right down the middle of Monoceros, through the legs of Gemini and through the middle of Auriga. The midline of the Milky Way is the galactic equator than I have indicated in the attached star chart. The Milky Way is not as noticeable in this part of the sky because we're looking outward away from the center of the Galaxy. In fact at a point between Auriga and Gemini (closer to Auriga) is the galactic anti-center. In the summer, when we look toward Sagittarius, we're looking toward the center of the Galaxy so the Milky Way there is brighter, fatter, and has many clusters in that direction.
The Milky Way is about 100,000 light years across. To get an idea of how our galaxy compares to the solar system, consider a scale model with Pluto's entire orbit fitting inside a quarter coin. The next star system (Alpha Centauri) would be about 84 meters (big steps) away. The Milky Way in this scale model would stretch across the western United States from Los Angeles to the city Pierre in the middle of South Dakota. Think about how many quarters could fit in the western United States and you would get an idea of what several hundred BILLION looks like.
Determining the structure of our galaxy is not an easy task because the solar system is stuck inside the Galaxy and we can only look in all different directions. Our situation is like you having to determine the layout of Bakersfield (or other home town) from just looking out on your front porch (or back porch) and not being able to move even across the street. The fact that you see a narrow band of stars tells you that our galaxy is shaped like a thin disk. If we lived in a more spherical galaxy, the stars would be distributed more uniformly across the sky. If we lived in an irregular galaxy, there would be patchier distribution of material in various parts of the sky instead of the narrow band of stars. There is a hint of a bulge in the direction of the Sagittarius constellation (toward the Galaxy center). Careful star counts and determining their distances shows hints of a spiral pattern in the disk. The interstellar dust limits our view a small section of the Galaxy roughly 6000 light years in any direction along the galactic plane.
The Milky Way Galaxy is disk-shaped with spiral arms in the disk. It has an elliptical bulge in the center with a bar-shaped distribution of gas/dust/stars going through the middle out of which the spiral arms extend and a spherical halo of stars that is denser closer to the Galaxy center. The disk of stars is about 100,000 light years across but only about 1000 light years thick (the dust layer is even thinner). The bar going through the middle of the bulge is about 25,000 by 4000 light years in dimension. Our solar system is about two-thirds of the way out from the center (27,000 light years from the center) in a spur off one of the major spiral arms. We were able to figure out our distance from the galaxy center in the early 1900s by measuring the distances to the globular clusters that swarm around the galaxy center like bees swarm around their hive. The fact that we did not see the globular clusters distributed around us told us that we were not at the galaxy center. More recently, we have fine-tuned the distance to the galaxy center using parallax measurements with the Very Long Baseline Array, made of ten radio telescopes hooked together electronically across the globe from Hawaii to St Croix in the Virgin Islands. Connecting telescopes together gives the array the resolution equivalent to a single telescope with a diameter that of the array. The Very Long Baseline Array has a resolution 50 times better than the Hubble Space Telescope, though the VLBA is observing in the radio band while the Hubble Space Telescope observes in the optical or visible band. See my online textbook Astronomy Notes at www.astronomynotes.com for more details of how we map the Milky Way, how we measure distances to stars, and how telescope arrays work.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com