Messier 51: Starry Starry Night

In 1845 William Parsons made a sketch of the Whirlpool Galaxy from a 72-inch reflector. Almost 40 years later, that particular sketch inspired the Dutch painter Vincent van Gogh to create his masterpiece – The Starry Night. Van Gogh was well aware of the astronomical discoveries made in his time. He knew space as a dynamic place – which appears beautifully in this cover image.

Influence of Astronomy on Art – here I have listed more art pieces like The Starry Night, which were influenced by astronomy!

Sketch of Whirlpool Galaxy, believed to be an inspiration for Starry Night; Source: Wikipedia

Galaxies in space often interact with each other if they ever happen to pass in close vicinity to one another. As you might have heard of – Milky Way and Andromeda collision, which will happen in the next 4.5 billion years. Scientists have found many cases of such interacting galaxies. One such interesting pair is – Whirlpool Galaxy.

Whirlpool Galaxy – processed by me. Data acquisition – HST

Introduction

The Whirlpool Galaxy is an interacting spiral galaxy. You can find it in the northern sky in the constellation Canes Venatici. The galaxy is easily observed and can be seen with binoculars.

General Facts

Whirlpool Galaxy lies 23 million light-years from Earth. Overall the galaxy is about 43% the size of the Milky Way. Its mass is estimated to be 160 billion solar masses. NGC 5195 is a dwarf galaxy that is interacting with the Whirlpool Galaxy. Both galaxies are located in the constellation Canes Venatici. Together, the two galaxies are one of the most widely studied interacting galaxy pairs.

Discovery

First discovered on October 13, 1773, by Charles Messier while hunting for objects that could confuse comet hunters, and was designated in Messier’s catalog as M51. Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain. In 1845, William Parsons using a 72-inch (1.8 m) reflecting telescope at Birr Castle, Ireland, found that the Whirlpool possessed a spiral structure, the first “nebula” to be known to have one. Whirlpool galaxy’s plane being face-on to our vision helped to observe those spiral arms around it.

These “spiral nebulae” were not recognized as galaxies until Edwin Hubble provided evidence that they were so far away that they must be entirely separate galaxies. Even the Andromeda galaxy was referred to as nebula in those days.

Interacting galaxy pair

Dwarf galaxy, NGC 5195, created the spiral structure and star formation in the Whirlpool Galaxy. Radio images of M51 unequivocally demonstrated that the Whirlpool and its companion galaxy are indeed interacting. However, we haven’t yet figured out if the dwarf galaxy is going through the plane of the Whirlpool Galaxy, coming from behind, or is it going away from the observer.

Spiral structure

Instead of messing it up – as one may expect, this little companion helps in stirring the galaxy and gives it this beautiful spiral appearance. This smaller galaxy may have passed through the main disk of M51 about 500 to 600 million years ago. In this proposed scenario, NGC 5195 came from behind M51 through the disk towards the observer and made another disk crossing as recently as 50 to 100 million years ago until it is where we observe it to be now, slightly behind M51.

The undisturbed galaxy will have circular orbits of stars orbiting around its center. But here smaller galaxy just stretches out the orbits and converts these circular orbits into tilted ellipses.

Illustration of possible orbits of stars in M51 and appearance of spiral arms.

Thus, Orion Arm of the Milkyway is just an appearance. Matter in spiral arms is not gravitationally bound.

Star formation

And where the boundary of these elliptic orbits come closer, there we see spirals – the arms of the galaxy. As a result, the gases in spiral arms become dense and the formation of stars begins in those arms. That’s why arms tend to lit up.

Here you can see how the arms are lit-up with new forming stars.

How to spot it in the night sky?

Follow the easternmost star of the Big Dipper, Eta Ursae Majoris, and going 3.5° southwest – you will find the Whirlpool Galaxy. Although not visible through unaided eyes, it can be seen through astronomical binoculars. Galaxy never sets for observers located above 43°N latitude; it reaches high altitudes throughout the northern hemisphere making it an accessible object from the early hours in winter through the end of the spring season, after which observation is hindered in lower latitudes.

Appearance

M51 is visible through binoculars under dark sky conditions, and it can be resolved in detail with amateur telescopes. Under dark skies, through a 150 mm telescope, M51’s spiral structure can be detected.

As is usual for galaxies, the true extent of its structure can only be gathered from inspecting photographs; long exposures reveal a large nebula extending beyond the visible circular appearance.

~AK

Published by Anand Krishna

Amateur astronomer and astrophotographer. Interested in astrophoto processing, astrostatistics, comet hunting, visual and radio astronomy.

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