# The Whirlpool Galaxy

How the Whirlpool Galaxy \(M51\) was found and how its spiral arms form and make stars

By greenlig

Topic: Space

[Open the interactive Jig](https://ask.jig.com/j/agCpxiam1y9h)

## Guide

### 1. A smudge by the Big Dipper

On 13 October 1773, comet hunter Charles Messier spotted a faint, fuzzy patch just off the end of the Big Dipper's handle\. Through his telescope it was a dim glow with no stars in it\. He listed it as object 51, so nobody would mistake it for a comet\.

It lies about 3\.5 degrees from Alkaid, the last star of the handle, in the small constellation Canes Venatici\.

### 2. A whirlpool of stars

Modern telescopes show what Messier could not: a whole galaxy, seen almost face\-on\. Its yellow core is crowded with old stars\. Two long spiral arms wind out from it, edged with dark lanes of dust and dotted with pink glowing clouds\.

It is about 23 to 31 million light\-years away: different methods disagree\.

### 3. A companion at the end of an arm

At the tip of one arm sits a smaller galaxy, NGC 5195, first seen by Pierre Méchain in 1781\. It lies a little behind the big galaxy, so the arm's dark dust crosses in front of its glow\.

Drag to turn the view and see the pair from the side\.

### 4. Traffic jams of stars

Stars do not ride along the arms\. Each one follows its own slightly oval orbit, and the ovals are turned a little more the farther out they are\. Where the orbits crowd together, stars and gas bunch up like cars in a traffic jam: that crowded zone is a spiral arm\.

Whether arms like these last for billions of years or come and go is still debated\.

### 5. The companion's pull

NGC 5195's gravity tugs on the big galaxy\. Computer models suggest it swung past once or twice in the last 600 million years, stretching the orbits into longer ovals\. Slide to see how a stronger pull sharpens the two arms\.

The companion's exact path and timing are uncertain: the models disagree\.

### 6. Where new stars are born

When a cloud of gas drifts into an arm's traffic jam, it is squeezed\. Its densest knots collapse into hot young stars, and their light makes the hydrogen around them glow pink\.

### 7. Beside our Milky Way

Our own galaxy is a spiral too, with a bar of stars across its middle and a disc about 100,000 light\-years wide\. The Whirlpool is a little smaller or about the same size: the answer depends on how far away it really is\.

The Whirlpool is about 77,000 to 100,000 light\-years across\.