Astronomical tidbits: Or, just how far away is Orion's Belt?

Nothing in space is at rest: all celestial objects are moving in relation to one another. The moon circles the Earth. The Earth, the other planets and their own moons, the asteroids, comets, and everything else in our solar system, make elliptical paths around the sun.

Our sun is in motion too.

The sun and all of its satellites – that is, the entire solar system - take about 230 million years to make one orbit around the Milky Way, our name for our spiral-shaped galaxy. How fast is that? We're moving at nearly 515,000 miles per second!

Did you know that at the center of the Milky Way is a black hole, an object so massive and dense that even light can't escape it? There are some 200 billion stars in the Milky Way, many, maybe even most, with their own planets, each stellar system tracing its own vast ellipse around that black hole.

The black hole's gravity pulls objects inward towards itself with unimaginable force, while the energy of motion of each circling object, its outward-pushing centripetal force, keeps that dance going for billions of years.

Here's a picture of our next-door neighbor, the Andromeda Galaxy:

Andromeda Galaxy

But the geometries of scale don't end there…

The Milky Way is itself in motion, on a curving path around the center of gravity of a "supercluster" of galaxies.

Galaxies upon galaxies...

There are 10 million superclusters; we call ours the Virgo Supercluster.

Ancient and medieval peoples, looking up at night sky, saw patterns in the stars. They gave those patterns – or constellations - names. We still use many of those names. One of the most famous constellations is Orion, the Hunter.  Can you see his or her shoulders and knees?

Orion, the Hunter

This astral hunter appears to be wearing a belt consisting of three stars close to one another and in a straight line. [You might remember that "Orion's Belt" was made famous to movie-goers in the original Men in Black film.]

Although people of many pre-technological cultures observed Orion and its distinctive asterism, and entertained their communities with stories about it, the names we know these three stars by today come from Arabic: Alnitak, Alnilam, and Mintaka. We know now that Alnitak itself is actually three stars.

But the stars that make up Orion's belt are not close together. Estimates vary, but Alnitak is about 800 light-years from Earth, where a light-year is the distance light travels in one year (a "year" is defined from our human perspective on Earth – that is, a year is 365 Earth days or the time it takes for Earth to complete one orbit around the sun). Using that definition, Alnilam is 1,340 light-years from Earth, while Mintaka is around 915 light-years away.

So, using a more common metric, one light year is 5,878,625,373,183.6 miles. In other words, the scales we're talking about are vast. But, you say, they look like they're right next to each other!

Appearances are deceiving.

Astronomers use a concept called "parallax" to estimate how far away stars are.

Parallax is the change in a star's apparent position when observed from Earth at two different places in our orbit around the sun. Take a look at this diagram to help you understand this concept – a picture is often worth a thousand words. Stars that appear to be close together from one point in our orbit may appear to be further apart from a different point. Knowing the angle between those points, and using geometry, astronomers can calculate how far away a celestial object is.

To an Earth-based observer, when objects in the sky appear close together, but are not actually proximate, astronomers say they are "in conjunction". Objects in conjunction are not limited to stars; planets can look close together in our night sky, too, even though they are millions of miles apart. Here's a list of upcoming planetary conjunctions:
Planetary Conjunctions

And here's a website which shows the sky visible from Worcester, as it changes night by night:
Worcester Night Sky

Here are some more spectacular images of our observable universe:

Crab Nebula	Saturn
Witch Head Nebula	M51spiral galaxy

Want to learn more? The library has many books on astronomy, astrophysics, and space science, both in print and as eBooks. Check out this list: WPL eBooks on astronomy

You can also browse the Overdrive and Hoopla apps for more free eBooks and eMagazines on astronomy; look for them on our website under Resources/eBooks & Digital Media, as shown below.

Gale in Context: Science, accessible from Resources/Online Databases has articles on astronomy and cosmology: see Gale

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