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Interacting Galaxies Galore!

25 Jul 2008, 04:00 UTC
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Interacting galaxies morph intowondrous new shapes.

Hubble celebrated its 18th anniversary by releasing a huge image gallery of interacting galaxies. Such galaxies pass close enough to each other that their mutual gravity can stretch and distort their shapes. Eventually, interacting galaxies merge together to form a single larger galaxy. However, since these interactions can take billions of years, how do we study them? And how do we make sense of the variety of strange shapes seen in these Hubble images?

Hubble press release:

Cosmic Collisions Galore!


Hubble was launched into orbit on April 24, 1990, aboard the space shuttle Discovery. However, there is a lot more to its history than just 18 years (so far) of cutting edge science. Take a look at Hubble Essentials for more of the story.

Here are the numbers behind the size vs. distance comparison, using baseballs as stars.

Let's take the stars first. The Sun is about 870,000 miles (1.4 million km) in diameter. The star Alpha Centauri is about 4.25 light-years away. A light-year is the distance light travels in one year at the speed of about 186,000 miles (300,000 km) per second, roughly 5.9 trillion miles (9.5 trillion km). That makes Alpha Centauri about 25 trillion miles (40 trillion km) distant. Hence the distance to Alpha Centauri is about 28 million times larger than the diameter of the Sun. I say "millions" in the podcast just to make it easy to remember.

Since a baseball is about 3 inches (7.6 cm) in diameter, a scale model of our Sun and Alpha Centauri would be two baseballs separated by 28 million times 3 inches (7.6 cm). Do the math, and you get about 1,300 miles (2,092 km). The distance between Baltimore (where the Space Telescope Science Institute is located) and Houston is about 1,250 miles (2,011 km). Not exact, but close enough for a good comparison.

The scientific visualization of the galaxy collision is based on a supercomputer simulation by two astronomers. The galaxies are represented in the computer by several hundred thousand particles that interact via the equations of gravity and hydrodynamics. The output of the simulation is just lists of positions, velocities, densities, temperatures, etc. for all the particles at each timestep of the simulation. Those data are then turned into pictures using custom visualization software designed to represent the physics of the simulation accurately. Commercial software, such as that producing the latest Hollywood computer graphics, is generally not that useful for scientific visualization, as it is optimized to produce fantasy instead of reality.

Image Notes

Hubble from Shuttle Mission STS-109
Credit: NASA

Orion star field
Credit: A. Fujii

Drawing used to represent the Milky Way
Credit: NASA, ESA, A. Feild (STScI)

Andromeda Galaxy
Credit: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF

Interacting Galaxies: Gallery and individual images of Arp 148, ESO 077, ESO 148, NGC 6240, UGC 09618, VV 705
Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Galaxy Collision: Simulation vs Observations
Credit: NASA, ESA, and F. Summers (STScI) Simulation Data: Chris Mihos (Case Western Reserve University) and Lars Hernquist (Harvard University)

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