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M51, Whirlpool Galaxy in Canes Venatici

TargetTypeR. A.Dec.ConstellationMagnitudeSize
M51 (NGC 5194)Spiral Galaxy13h 29m 53s+47° 11′ 43″Canes Venatici+7.914×12 arcmin

Overview

Messier discovered what he called ‘A very faint nebulae without stars’ in October 1773. We since discovered the concept of galaxies – there was no such thing in Messier’s time – and know that the Whirlpool is a magnificent, face-on, archetypal example of this structure. The companion galaxy, NGC 5105, was discovered by Messier’s friend Pierre Mechain eight years later.

Lord Rosse was the first to see the spiral form of M51 in 1845 but it would be almost 80 years later before we discovered that this was a galaxy and not a ‘spiral nebula’ in 1923.

We now know that this circumpolar object (meaning that it never sets for most northern hemisphere observers) is more than 30 million light-years away from us and that its true size is something in the order of 110,000 light-years in diameter. Its central region is about 400 million years old.

With the advent of radio astronomy, we also discovered that these two galaxies are physically interacting with each other, i.e. it is more than a visual effect.

M51 is thought to be a little bit smaller than our home galaxy, the Milky Way, which is why gazing at it comes with more than the usual sense of awe and wonder. Its mass is in the order of 160 billion suns, which is only one-tenth that of the Milky Way, meaning the Whirlpool is much less densely packed with stars than the Milky Way.

Two galaxies in one. The larger is M51, the Whirlpool Galaxy, the smaller is NGC 5195. They are part of the same system. (Source)

Finder Charts

Our first finder chart for M51, giving the view at 9 p.m. in mid-September, shows that it lies just beyond the end of the Big Dipper’s (Plough’s) handle. It’s displayed as a green circle in the image below and the Big Dipper is circled in orange.

This SkySafari 6 finder chart shows stars to magnitude 5.0 and the moon and planets are not shown.

[M51-1] M51 is easy to locate near the last star in the Big Dipper’s handle. Click for full-screen.

The final star in the Big Dipper’s handle is Eta Ursae Majoris (Alkaid). It’s a bright, magnitude 2 star, easily visible to the naked eye even under moderate light pollution. M51 is less than 4° southeast of there, which is about the width of three fingers held at arm’s length.

[M51-2] The location of M51 in relation to the Big Dipper (Plough) is shown. Click for full-screen.

Our final chart shows stars to magnitude 7.0 and you can see 1° circles in blue, showing the star hop from the magnitude five star 24 Canum Venaticorum. If we treat the line joining Alkaid to Mizar as one side of a right-angled triangle (dashed orange lines) it can make M51 easier to track down. Another way to think about it is this: if Alkaid is at 12 o’clock in your finderscope, and Mizar is at three o’clock, we’ll find M51 at 8 o’clock.

[M51-3] To find M51, work your way from 24 CVn Remember, the Whirlpool is easily seen in binoculars and magnifying finderscopes. Download below or click for full-screen.

The individual telescope views below give a better view of how this galaxy will appear in your eyepiece.

Individual Telescope Views

The following views will help you find M51 in different telescope types by presenting the images as your telescope will show them. Stars are shown to magnitude 11.0 . The large circle around the galaxy is a 1° field of view. The first image in each pair is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Each image can be clicked on for a full-screen version.

Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope

[M51-4]
[M51-5]

Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal

[M51-6]
[M51-7]

Mirrored View – Refractors and Cassegrain models with a star diagonal show this view

[M51-8]
[M51-9]

Observation

You’re going to see the bright core and fainter halo of M51 with moderate magnification. However, like most galaxies, the Whirlpool is sensitive to light pollution, so you’ll always get better results under a darker sky.

A small telescope struggles to show the spiral arms, but a larger aperture will make them easier to distinguish. What the smaller model will reveal is the second galaxy, NGC 5195, as a second glowing core to the side of the main show. We’ll need a large aperture to reveal the light from the connecting stars and gases between the two of them.

Use your larger aperture to discover the mottled nature of the light, especially as t brightens towards the core. This is suggestive of the spiral arms that great conditions allow us to tease out. What we can observe in less-than-perfect conditions is that this galaxy is not quite the perfect circle, but is slightly squished along one axis.

However you choose to observe it, this is a stunning object to behold and a mirror or our own galaxy for anyone looking back at us from 30 million light-years away.


PDFs for Printing

Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [M51-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.

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