Here is your weekly reminder of Psalm 19 — colliding galaxies, Arp 274.

Galaxy collisions are a testament to the strange way in which space is scaled. The universe is a relatively crowded place on the galactic scale, which is why these collisions are fairly common. But when galaxies crash into each other, the stars in them are so far apart from each other that the stars themselves usually don’t collide.

Think of it this way. If you were to draw a 1 cm dot that represented the Sun, the nearest star to the Sun (Alpha Centauri, ~4 light-years away) would be a slightly larger dot about 400 km away. That’s how much space there is between the stars.

Now, if you were to draw a 1 cm dot that represented the Milky Way Galaxy, the nearest galaxy to ours (Andromeda, ~2.5 million light-years away) would only be 25 cm away.

That’s why galaxies often collide, but stars usually don’t. However, the gas and dust that is inside galaxies does collide, and this leads to a brief period of intense star formation as the galaxies gravitationally tear each other apart. Once this violent dance settles down, a newly formed galaxy remains.

Galaxy collisions take hundreds of millions of years to play out, so what we’re seeing with images like the one above are cosmic snapshots of collisions. Astrophysicists use supercomputer simulations to hugely speed up the process and explore what a full collision would look like.

Image credit: NASA, ESA, M. Livio (STScI) and the Hubble Heritage Team (STScI/AURA).

Here is your weekly reminder of Psalm 19 — Saturn’s moon, Enceladus.

Enceladus is the sixth-largest of Saturn’s 62 moons. At 500 km diameter, it is dwarfed by Saturn’s largest moon, Titan (5,100 km) and Earth’s Moon (3,500 km). It was discovered in the late 18th century by the German-English astronomer, William Herschel, whose son, John, named it. Not much was known about Enceladus until the Voyager missions studied the moon in the 1980s. The Cassini mission followed in 2005 to study Saturn and its moons in greater detail. The above image was taken during this latest mission.

The surface of Enceladus is comprised of clean ice (as opposed to “dirty” ice, which contains rock, dust, and organic compounds) that reflects most of the sunlight that reaches it. Enceladus has an active surface, with over 100 geysers spewing water vapor into the rings of Saturn. Last year, Cassini found evidence of a subsurface ocean beneath the icy surface. The Cassini spacecraft is scheduled to fly through one of Enceladus’ geyser plumes in the hope that it will reveal the chemical makeup of its liquid ocean.

Image credit: NASA/JPL.

Here is your weekly reminder of Psalm 19 — our galactic home, the Milky Way.

This graceful arch across the sky is our home, as seen from within the Galaxy itself.

Have you ever wondered why it’s called the Milky Way? The ancient Greeks called it the “milky circle,” because they thought it looked like milk spilled across the sky. (The Greek word for “milky” is galaxias, which makes the name Milky Way Galaxy a little tautological.)

The Milky Way is a spiral galaxy, about 100,000 – 150,000 light-years across. It contains an estimated 200-400 billion stars, including our Sun. The Solar System is located about 30,000 light-years away from the center of the galaxy at the edge of the Orion arm (see image below). Incidentally, did you know there are stars between the arms in a spiral galaxy? Quite a few actually, but many of them are not visible because they are dimmer than the very luminous massive stars that tend to bunch up in the arms.

The Milky Way has a supermassive black hole in its center, just like every other galaxy that’s been observed. It weighs in at about 4 million times the mass of the Sun, which may sound like a lot, but is kind of “meh” as far as supermassive black holes go (some of these giant black holes tip the cosmic scales at 10 billion times the mass of the Sun). If you wanted to look in the sky in the direction of this black hole, called Sagittarius A*, you would look at the Milky Way in the direction of the constellation Sagittarius. You wouldn’t see anything that suggests a black hole, especially because it’s smaller in size than Mercury’s orbit around the Sun, but it’s sort of fascinating to know that it’s there nevertheless.

Fraser Cain explains what we’re actually looking at when we observe the Milky Way in the sky:

Milky Way image credit: Steve Jurvetson. Milky Way schematic credit: Sky & Telescope magazine.

Here is your weekly reminder of Psalm 19 — star forming region, LH 95.

LH 95 is a stellar nursery, a region in which star formation is actively occurring, in the Large Magellanic Cloud (LMC). The LMC is a small, irregular satellite galaxy orbiting the Milky Way, but visible only from the Southern Hemisphere. LMC’s close proximity allows detailed views of stars and nebulae in a galaxy outside of our own.

Astronomers have identified thousands of baby stars in their initial stages of development in this nursery, providing a detailed picture of how star formation in the early Milky Way likely occurred. The blue color in LH 95 is starlight from very large, hot stars reflecting off hydrogen gas. This glowing gas is surrounded by the cold, dusty molecular gas out of which new stars form.

Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration.

Here is your weekly reminder of Psalm 19 — a “supermoon” lunar eclipse.

Last Sunday, many of us were treated to a rare combination of a lunar eclipse and a “supermoon.” A supermoon occurs when a full moon phase coincides with the Moon being at its closest point in its slightly elliptical orbit around the Earth, making our lunar companion look slightly larger (~14% in diameter) in the sky than normal. What really makes a supermoon “super” is its increased brightness — owing to its closeness to the Earth, a supermoon is 30% brighter than a regular full moon.

A lunar eclipse occurs when the Earth moves between the Sun and the Moon, blocking out the sunlight that normally reflects off of a full moon.

When I teach introductory astronomy, the students who are really paying attention will ask why we don’t always get a lunar eclipse during a full moon phase. The answer is, the plane of the Moon’s orbit (outlined with the green circle above) is slightly tilted with respect to the Earth’s orbital plane (outlined in blue), so that most of the time the Earth does not block light coming from the Sun. Rarely, we’ll get the Sun, the Earth, and the Moon lining up when the Moon is in the Earth’s orbital plane, and that’s when we experience a lunar eclipse.

The next time a supermoon will coincide with a lunar eclipse is in the year 2033.

Supermoon lunar eclipse photo credit: Dina Rudick (Boston Globe). Lunar eclipse schematic credit: Wikipedia.

Here is your weekly reminder of Psalm 19 — the planet Mercury.

Mercury is the smallest planet, as well as the planet closest to the Sun. It has a remarkably long day — a Mercury day lasts 88 Earth days — and a relatively short year (116 Earth days). Because of the peculiar ratio of its orbital period to its rotational period, a hypothetical observer on Mercury would experience only one day for every two years.

Mercury has no atmosphere, so the range of surface temperatures is extreme — -280 F during the night (the part of Mercury that faces away from the Sun) and up to 800 F during the day (the part of Mercury that faces toward the Sun).

Mercury has the most eccentric orbit in the Solar System, which means that out of all the planets, its orbit is the most like an ellipse. The part of its orbit closest to the Sun (the perihelion) precesses, which means Mercury’s orbit spirals around the Sun like a spirograph. Newton’s version of gravity could account for some of this precession, but not all of it. The reason for the discrepancy remained a mystery for centuries, until Einstein formulated his General Theory of Relativity, which explained the precession in terms of the way the Sun warps space around Mercury.

Image credit: NASA.