Alan Stahler: Farthest thing we can see without glass
February 7, 2018
Nothing moves faster, in the vacuum of space, than light: light can zoom around the world seven times, in under a second.
But the universe is huge, and travel between stars, even at light speed, takes years. Starlight from Sirius — brightest star in the sky — takes over eight-and-a-half years to reach us. (Find Sirius by extending Orion's belt to the left).
Flying through space for a year, light covers nearly six trillion miles (a trillion is one with 12 zeros). This distance — nearly six trillion miles — equals one light-year. Sirius lies eight-and-a-half light-years from Earth.
As cosmic distances go, eight-and-a-half light-years is pretty darn close, which is why Sirius shines so bright in our sky. Most stars are more distant.
The constellation Andromeda hosts the farthest object we can see with the naked eye: A galaxy that shines with the combined light of hundreds of billions of stars. The Andromeda Galaxy ("Andromeda," for short) lies two-and-a-half million light-years from Earth.
To the naked eye, Andromeda is a faint fuzzball. If skies are clear this Saturday night, we'll get a much better view of Andromeda through telescopes, when local astronomers meet at the junction of State Route 49 and the old Downieville Highway, beginning at 6 p.m. All are welcome — dress warm.
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Earth's spin carries Northern California around at 800 miles an hour. Only gravity keeps us from being flung out into space.
Galaxies also spin, and it's gravity that keeps their stars from flying off into space. But when we count the stars in the Milky Way, or in Andromeda, and calculate the gravity of those stars … gravity's too weak to hold the galaxies together. The stars of the Milky Way and of Andromeda should fly off.
What holds it all together?
Perhaps our galaxy, and Andromeda, are held together by the gravity of some strange sort of matter that neither emits nor reflects any light: Dark Matter.
Most planets, including Earth, bulge at the equator. That bulge at the beltline tends to pull the moons of planets into line with their equators.
A satellite is any object that orbits another. The moon is a satellite of Earth; Earth is a satellite of the sun. Orbiting our Milky Way galaxy, and orbiting Andromeda, are small, satellite galaxies.
If Dark Matter really holds the galaxies together, theory says it should be shaped as a sphere, toward the center of the galaxy — it should not bulge at the beltline, should not pull satellite galaxies into orbits around the "equators" of galaxies.
And yet, satellite galaxies of the Milky Way, and of Andromeda, do orbit our galactic "equators." We have here conflicting evidence: Dark Matter exists … Dark matter does not exist.
The Andromeda Galaxy is twice the size of the Milky Way, the result, perhaps, of a collision that merged two galaxies into one, at the same time providing Andromeda its satellite galaxies.
Andromeda won't always be the farthest thing we can see with the naked eye — it's coming closer by the moment, heading for yet another collision, billions of years from now, this time with our own Milky Way.
Besides Andromeda and its satellite galaxies, this Saturday, we'll watch the International Space Station emerge from Earth's shadow, in the northwest, at 7:01 p.m., and fly nearly overhead. With over half-an-acre of glass-covered solar panels, the station reflects a lot of sunlight. Watch for the station from home, if you can't join us — simply look straight up, just after 7:01 p.m. A second opportunity occurs at 6:01 p.m. Tuesday, Feb. 13. (Thanks to avid station-watcher Frank Sobrero, for goosing me to put out the heads-up).
Al Stahler enjoys sharing nature with students of all ages. His science stories can be heard on radio station KVMR (89.5 FM). He may be reached at email@example.com
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