Milky Way Galaxy
Our sun is about 26,000 light-years from the center of the Milky Way Galaxy, which is about 80,000 to 120,000 light-years across (and less than 7,000 light-years thick). We are located on on one of its spiral arms, out towards the edge. It takes the sun (and our solar system) roughly 200-250 million years to orbit once around the Milky Way. In this orbit, we (and the rest of the Solar System) are traveling at a velocity of about 155 miles/sec (250 km/sec).
Since we’re inside the Milky Way Galaxy and we’ve never sent a spacecraft outside our Galaxy, we have no photographs of the Milky Way Galaxy. Radio telescope data does, however, let us know a lot about it.
The arms of the Milky Way are named for the constellations that are seen in those directions. The major arms of the Milky Way galaxy are the Perseus Arm, Sagittarius Arm, Centaurus Arm, and Cygnus Arm; our Solar System is in a minor arm called the Orion Spur. The central hub (or central bulge) contains old stars and at least one black hole; younger stars are in the arms, along with dust and gas that form new stars.
The great rift is a series of dark, obscuring dust clouds in the Milky Way. These clouds stretch from the constellation Sagittarius to the constellation Cygnus.
The Milky way Galaxy is just one galaxy in a group of galaxies called the Local Group. Within the Local Group, the Milky Way Galaxy is moving about 300 km/sec (towards the constellation Virgo). The Milky Way Galaxy is moving in concert with the other galaxies in the Local Group (the Local Group is defined as those nearby galaxies that are moving in concert with each other, independent of the “Hubble flow” expansion).
Not counting transient events such as gamma-ray bursts, the brightest object in the gamma-ray sky is the plane of our Milky Way Galaxy. This glow results from a vast sea of cosmic-ray particles slamming into interstellar gas and dust, generating gamma rays. In fact, 75% of the gamma rays in our galaxy come from these cosmic-ray interactions. This bright gamma-ray glow gives the GLAST science team a golden opportunity to study the structure, composition, and dynamics of the interstellar material that pervades our home galaxy.
But as Large Area Telescope science team member David Thompson of NASA Goddard, explains, “It’s not easy to understand something when you’re in the middle of it.” Adding to the complexity is the fact that our galaxy is filled with many different types of particles and energy sources, including protons, electrons, electromagnetic radiation, magnetic fields, and so forth — most of which have not been accurately measured.
To study our galaxy, theorists create models of how these different particles interact with magnetic fields in different locations and with different strengths. Astronomers can then compare these models to actual observations made at radio, infrared, optical, ultraviolet, and X-ray wavelengths to see how well they match the data. The LAT will contribute vital data that will enable theorists to constrain and improve their models.
For example, the EGRET instrument on NASA’s Compton Gamma-ray Observatory saw hints that there may be clumps of gas in our galaxy that are not seen by radio telescopes. GLAST observations of the galactic plane should be able to help astronomers pin down whether or not these clumps are real. They might also reveal changes in the interstellar medium due to recent supernovae.
Having an accurate model of gamma-ray production within our galaxy is not only important in its own right, it is vital for the measurement of localized gamma-ray sources. The sources are seen against the bright background of the Milky Way glow. If the galaxy is not modeled correctly, then information about other objects could be distorted. As GLAST Program Scientist F. Rick Harnden Jr. notes, “The same gamma rays that measure galactic structure are also a background for other observations.”