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Exploring the Whirlpool Galaxy, M51

written by: •edited by: George Adcock•updated: 6/1/2011

There are probably no more esthetically pleasing structures in nature than a spiral, and this is especially true when one looks at the many different types of galaxies. The Whirlpool galaxy, M51, is an excellent example of a spiral galaxy. Read on to learn more about this island universe.

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    Getting To Know The Whirlpool Galaxy

    Take a close look in the constellation, Canes Venatici, which lies below the Big Dipper, and if you look below the last star of the handle of the dipper you will find Messier object, M51, or the Whirlpool galaxy. The galaxy also has the New General Catalog classification, NGC 5194. A quick look at the enhanced image from the Hubble Space Telescope below, easily demonstrates why it is known as the Whirlpool galaxy. Also evident is its companion, NGC 5195, which is intimately connected to M51.

    The Whirlpool Galaxy From Hubble (NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA)) 

    This Hubble image clearly shows the dark dust lanes that lace the arms of the galaxy. The blue color in the arms is due to the hot, blue stars (type O and B stars) that have formed in these dust-rich regions of the galaxy. The pinkish knots are star forming regions – stellar nurseries. These are creating stars that will shine on long after the short-lived blue stars that populate the arms have consumed the last of their fuel.

    The image also clearly shows the connection between M51 and its companion, NGC 5195, as it pulls on the arm of M51. For many years it was unknown if it was interacting with M51 or just in the line of sight behind M51. It wasn’t until observations from radio telescopes revealed that they are interacting with each other, and this interaction may have generated the density waves that contributed to the spiral structure of the Whirlpool galaxy.

    Radio Image of M51 from NRAO (Image courtesy of NRAO/AUI and Juan M. Uson, NRAO) 

    This composite image above of visual and radio wavelengths from the National Radio Astronomy Observatory, using the Very Large Array (VLA), shows the structure of the interacting pair of galaxies. The blue sweeping curves and envelope that surrounds the galaxy is from neutral hydrogen atoms, and indicates that these two galaxies have been gravitationally entwined for a long time.

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    Facts About The Whirlpool

    The Whirlpool galaxy is a classic spiral galaxy (type Sc) seen full-on, as opposed to the edge-on view typified by the Sombrero galaxy. The galaxy is about the same size as the Milky Way, 100,000 light-years across. At a distance of about 26 million light-years (I have seen distance estimates from 22 - 37 million light years on the Internet.), it shines at a magnitude of 8.4, outside the range of naked eye observation, but easily seen in a telescope (the larger the aperture, the greater the structural detail that will be visible). Interestingly enough, Messier never knew the true beauty of the galaxy, as he was only able to resolve it as a nebulous cloud in 1774. It wasn’t until 1845 that the spiral structure of the galaxy was discerned and documented by the Earl of Rosse, William Parsons, using a 72 inch reflector. A sketch by Parsons of the galaxy can be seen below. This is a very interesting sketch considering that he made it without the benefit of the active optics we have today, and probably didn't understand how the atmosphere really affected the images they saw through the eyepiece.

    Sketch of the Whirlpool Galaxy by William Parsons (Wikimedia Commons) 

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    M51 From Other Parts Of The Spectrum

    Viewing the Whirlpool galaxy in other parts of the spectrum reveals more interesting details of this beautiful galaxy. In the Spitzer SpaceSpitzer infrared image of M51 (NASA/JPL-Caltech/R. Kennicutt (Univ. of Arizona)/DSS)  Telescope image to the right, comparing the visual to the infrared spectrum, it can be seen that the arms are connected by a lattice of dust. This structure is unusual, as it was expected that the arms would be well defined, with the dust confined to the arms. Another interesting component of the image is that the companion galaxy NGC 5195 shows very little dust, which indicates a population of much older stars. This is also evident from the yellow/white color seen in the visual images, showing very few hot, young stars. Remember that the dust re-radiates energy it has absorbed from the stars in the longer wavelengths of infrared energy.

    In the image of M51 and its companion to the left, provided by the Chandra X-ray observatory, the individual galactic M51 in X-rays (Credit: NASA/CXC/U.Md/A.Wilson et al) cores show prominently. These clouds of gas at very high temperatures, on the order of millions of degrees, are energized by jets from a massive black hole at the core. The other dots showing in the image may be from neutron stars or smaller black holes that reside in binary star systems in the galaxy.

    The Hubble Space Telescope provided a very interesting image of the black hole at the center of the Black hole at the Whirlpool's core (H. Ford (JHU/STScI), the Faint Object Spectrograph IDT, and NASA) Whirlpool galaxy, which can be seen on the right. "X" marks the spot of the black hole. It is theorized that two criss-crossing rings of dust absorb energy being emitted from dust and gas plummeting into the black hole. The diameter of the rings is about 100 light-years.

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    M51, NGC 5194 Or Just The Whirlpool Galaxy

    The Whirlpool galaxy is visually stunning no matter what part of the electromagnetic spectrum one views it in. Looking at these images makes me hunger to see just what our Milky Way looks like from a similar vantage point. It also makes me wonder if there isn’t some alien intelligence looking at our Milky Way, and marveling at its structure, all the while wondering just what their galaxy looks like.

    Check out the sites for the Hubble Space Telescope, the Spitzer Space Telescope, the Chandra X-ray observatory and the National Radio Astronomy Observatory for more images and detailed information on the Whirlpool galaxy.

    I'll leave you with one more image of the Whirlpool galaxy, showing a composite of several images across the spectrum.

    Composite Image of the Whirlpool galaxy. 

    (Credit: X-ray: NASA/CXC/Wesleyan Univ./R.Kilgard et al; UV: NASA/JPL-Caltech; Optical: NASA/ESA/S. Beckwith & Hubble Heritage Team (STScI/AURA); IR: NASA/JPL-Caltech/ Univ. of AZ/R. Kennicutt)