Leviathan's Deep Sky Object of the Week #11:
The Southern Pinwheel Galaxy - Messier 83
Constellation: Hydra
Right ascension: 13h 37m 00.9s
Declination: -29° 51′ 57″
Redshift: 513 ± 2 km/s
Distance: 14.7 Mly (4.5 Mpc)
Type: SAB(s)
Apparent dimensions(V): 12′.9 × 11′.5
Apparent magnitude (V): 7.54
Right ascension: 13h 37m 00.9s
Declination: -29° 51′ 57″
Redshift: 513 ± 2 km/s
Distance: 14.7 Mly (4.5 Mpc)
Type: SAB(s)
Apparent dimensions(V): 12′.9 × 11′.5
Apparent magnitude (V): 7.54
This spectacular spiral galaxy is known to astronomers as Messier 83. Colloquially, it is also called the Southern Pinwheel due to its similarity to the more northerly Pinwheel galaxy Messier 101. M83 is classified as a starburst galaxy - a galaxy where a great deal of star formation is taking place. The image above shows us, in spectacular detail, the numerous star clouds full of bright young blue giant stars whose births may have been triggered by massive gravitational interactions between M83 and one of several nearby companion galaxies. In the image above we can also see a fantastic amount of gas and dust structure in what many people think of as our own Milky Way galaxy's smaller cousin.
Living in the middle of the Milky Way's disk, we see our galaxy only from an obstructed vantage point that is both inside-out and edge-on. We see Messier 83 nearly face-on, giving us a chance to really map out its disk in great detail. This information helps astronomers figure out what our own galaxy would look like if we could warp out to a better vantage point.
Like the Milky Way, Messier 83 is classified as a barred spiral galaxy due to the bar-like pattern of stars that run through its center. This bar region is more interesting in the infrared (see below) since we can also see the open s shaped curve of dust (red) cutting through the more linear stellar bar (blue-cyan).
Living in the middle of the Milky Way's disk, we see our galaxy only from an obstructed vantage point that is both inside-out and edge-on. We see Messier 83 nearly face-on, giving us a chance to really map out its disk in great detail. This information helps astronomers figure out what our own galaxy would look like if we could warp out to a better vantage point.
Like the Milky Way, Messier 83 is classified as a barred spiral galaxy due to the bar-like pattern of stars that run through its center. This bar region is more interesting in the infrared (see below) since we can also see the open s shaped curve of dust (red) cutting through the more linear stellar bar (blue-cyan).
This arc of inner dust connects up with the more tightly wound spiral arms in the outer disk, seen here as bright green-red ridges. Between the main spiral arms we also see a complex webbing of dust that permeates the entire disk.
Another interesting note is that, just one year ago, the Chandra X-ray Satellite detected an ultra-luminous x-ray source (ULX) near the supermassive black hole at the center of the galaxy. For several short periods of time, over the last year, this source has brightened the total X-ray output of the black hole region by over 3000x! Astronomers now interpret these flucuations as episodes where the accrection disk around the supermassive black hole is being impacted by gases being ripped from the outer layers of a red giant star orbiting near the accretion disk's edge. So we have actual evidence of a star being eaten by a supermassive black hole. (A similar situation occured in the Andromeda galaxy (M31) just recently).
In 2006, astronomers using a high resolution infrared spectrometer on the Gemini South telescope detected a "high mass concentration" hidden in one side of the bar near the center of the galaxy. Models suggest that this could be a second nucleus from a smaller galaxy that has been swallowed up and incorporated into the larger barred spiral. The nucleus of the intruder body has an estimated mass of about 16 million times the mass of the Sun, compared to 2 million solar masses for the optical "main" nucleus. The two nuclei are about 100 parsecs apart and are probably harboring black holes. Numerical modeling conducted by the team suggest that the two nuclei would coalesce to form a single massive core in about 60 million years.
Another interesting note is that, just one year ago, the Chandra X-ray Satellite detected an ultra-luminous x-ray source (ULX) near the supermassive black hole at the center of the galaxy. For several short periods of time, over the last year, this source has brightened the total X-ray output of the black hole region by over 3000x! Astronomers now interpret these flucuations as episodes where the accrection disk around the supermassive black hole is being impacted by gases being ripped from the outer layers of a red giant star orbiting near the accretion disk's edge. So we have actual evidence of a star being eaten by a supermassive black hole. (A similar situation occured in the Andromeda galaxy (M31) just recently).
In 2006, astronomers using a high resolution infrared spectrometer on the Gemini South telescope detected a "high mass concentration" hidden in one side of the bar near the center of the galaxy. Models suggest that this could be a second nucleus from a smaller galaxy that has been swallowed up and incorporated into the larger barred spiral. The nucleus of the intruder body has an estimated mass of about 16 million times the mass of the Sun, compared to 2 million solar masses for the optical "main" nucleus. The two nuclei are about 100 parsecs apart and are probably harboring black holes. Numerical modeling conducted by the team suggest that the two nuclei would coalesce to form a single massive core in about 60 million years.
While Messier 83 is about 15 million light years away, it is actually one of the closest barred spiral galaxies in the sky. This gives astronomers an excellent chance to study a galaxy that, although half as big, seems very similar in structure to our own Milky Way galaxy.
Here are some observation notes I made while observing this galaxy from a dark sky location along the north shore of lake Erie with my 8" f 6.7 newtonian telescope on a moonless night many years ago using a magnification of 175x:
"Wow, what a beautiful spiral galaxy! It is very bright, large, and elongated, with two fairly tightly wound spiral arms - one of which has a more severe bend in it than the other. The core of the galaxy is quite bright and a few dusty wisps of hazy light can be barely perceived in the arms (dust lanes?). Averting my eye towards the side of the field of view through the eyepiece really helps improve the brightness and detail visible in the arms. After locating it and checking its position over the barrel of the telescope, it has become pretty easy to spot in the finderscope. A third arm appears to be jutting out of the less severely curved are about 1/3 -1/2 of the way around it from the center."
In a 20" f/4 dobsonian, using a 13mm Nagler and Parracor, Steven Coe reports, "spiral arms obvious, core blazing with 3 to 4 arc sec in size, many bright areas in arms (probably giant star clusters or HII regions).
Steven also reports that when viewed from the Texas Star Party through a 36" f/5 with a 20mm Nagler eyepiece, the "spiral was easy; averted vision makes the spiral structure more obvious and increases size 30%. Many small knots in the arms. Amazing amount of detail. Core a very light yellow."
"Wow, what a beautiful spiral galaxy! It is very bright, large, and elongated, with two fairly tightly wound spiral arms - one of which has a more severe bend in it than the other. The core of the galaxy is quite bright and a few dusty wisps of hazy light can be barely perceived in the arms (dust lanes?). Averting my eye towards the side of the field of view through the eyepiece really helps improve the brightness and detail visible in the arms. After locating it and checking its position over the barrel of the telescope, it has become pretty easy to spot in the finderscope. A third arm appears to be jutting out of the less severely curved are about 1/3 -1/2 of the way around it from the center."
In a 20" f/4 dobsonian, using a 13mm Nagler and Parracor, Steven Coe reports, "spiral arms obvious, core blazing with 3 to 4 arc sec in size, many bright areas in arms (probably giant star clusters or HII regions).
Steven also reports that when viewed from the Texas Star Party through a 36" f/5 with a 20mm Nagler eyepiece, the "spiral was easy; averted vision makes the spiral structure more obvious and increases size 30%. Many small knots in the arms. Amazing amount of detail. Core a very light yellow."
Leviathan's Deep Sky Object of the Week #11: Edge-On Spiral Galaxy
The Whale Galaxy
Other designations: Whale Galaxy, UGC 7865, PGC 42637, Arp 281,
Caldwell 32
Constellation: Canes Venatici
Right ascension: 12h 42m 08.0s
Declination:+32° 32′ 29″
Redshift: 606 ± 3 km/s
Distance: 27 +/- 3.1 Mly
Type: SB(s)d
Apparent dimensions: (V) 15′.5 × 2′.7
Apparent magnitude: (V) +9.8
Caldwell 32
Constellation: Canes Venatici
Right ascension: 12h 42m 08.0s
Declination:+32° 32′ 29″
Redshift: 606 ± 3 km/s
Distance: 27 +/- 3.1 Mly
Type: SB(s)d
Apparent dimensions: (V) 15′.5 × 2′.7
Apparent magnitude: (V) +9.8
NGC 4631 is a magnitude 9.8 edge-on spiral galaxy in Canes Venatici. The lens or disk shape of this galaxy is distorted with a thicker wedge at one end. In photos and visual observations with medium- to large-size telescopes, it resembles the view of a blue whale and is therefore known as “the Whale galaxy.” The galaxy is approximately thirty million light years away and is part of the NGC 4631 galaxy group. Its closest companion is magnitude 13.1 galaxy NGC 4627, an elliptical dwarf also known as “The Calf.” Together, they are known popularly as “The Whale and Calf.”
NGC 4631 contains a central starburst region, which is an area of intense star formation. The strong star formation is evident in the emission from ionized hydrogen and interstellar dust heated by the stars formed in the starburst. The most massive stars that form in star formation regions only burn hydrogen gas through fusion for a short period of time, after which they explode as supernovae. So many supernovae have exploded in the center of NGC 4631 that they are blowing gas out of the plane of the galaxy. This superwind can be seen in X-rays and in spectral line emission. The gas from this superwind has produced a giant, diffuse corona of hot, X-ray emitting gas around the whole galaxy. (Click on the image below for a closer view of this activity by the Hubble Space Telescope.)

Here, a profusion of starbirth lights up the galactic centre, revealing bands of dark material between us and the starburst. The galaxy’s activity tapers off in its outer regions where there are fewer stars and less dust, but these are still punctuated by pockets of star formation. Image by the Hubble Space Telescope Science Institute.
NGC 4631 has a nearby companion dwarf elliptical galaxy, NGC 4627. NGC 4627 and NGC 4631 together were listed in the Atlas of Peculiar Galaxies as an example of a "double galaxy" or a galaxy pair.
NGC 4631 and NGC 4627 are part of the NGC 4631 Group, a group of galaxies that also includes the interacting galaxies NGC 4656 and NGC 4657 (which together are also known as the 'Hockey Stick' galaxy. However, exact group identification is problematic because this galaxy and others lie in a part of the sky that is relatively crowded. Estimates of the number of galaxies in this group range from 5 to 27, and all studies identify very different member galaxies for this group.
NGC 4631 and NGC 4627 are part of the NGC 4631 Group, a group of galaxies that also includes the interacting galaxies NGC 4656 and NGC 4657 (which together are also known as the 'Hockey Stick' galaxy. However, exact group identification is problematic because this galaxy and others lie in a part of the sky that is relatively crowded. Estimates of the number of galaxies in this group range from 5 to 27, and all studies identify very different member galaxies for this group.
Observer's Notes:
As mentioned above, a large number of stars in the central core have apparently exploded as supernovae in a short time frame and they are ejecting massive amounts of gas from the core, creating a superwind. Unfortunately, this wind can only be seen in X-ray photos. For visual observing, there is no indication this is happening. However, what is noticeable, at high power in telescopes with apertures of 8 inches or larger, is small tufts and wisps of nebulosity extending irregularly and mostly perpendicular to the spiral's dust lane. The dust lane itself is differentiated and mottled in its appearance - sometimes darker and thinner near one end and then fainter and broken towards the other. Small bright patches pop in and out on either side of the central axis -these are giant star burst regions within the galaxy - clouds filled with newly formed bright blue giant stars. On crisp nights of good seeing, the detail that can be seen at higher powers is significant, particularly with apertures larger that 14 inches.
As mentioned above, a large number of stars in the central core have apparently exploded as supernovae in a short time frame and they are ejecting massive amounts of gas from the core, creating a superwind. Unfortunately, this wind can only be seen in X-ray photos. For visual observing, there is no indication this is happening. However, what is noticeable, at high power in telescopes with apertures of 8 inches or larger, is small tufts and wisps of nebulosity extending irregularly and mostly perpendicular to the spiral's dust lane. The dust lane itself is differentiated and mottled in its appearance - sometimes darker and thinner near one end and then fainter and broken towards the other. Small bright patches pop in and out on either side of the central axis -these are giant star burst regions within the galaxy - clouds filled with newly formed bright blue giant stars. On crisp nights of good seeing, the detail that can be seen at higher powers is significant, particularly with apertures larger that 14 inches.
Smaller telescopes show some features as well. In a 4 inch refractor at Starfest a few years ago, I recorded a few notes on my observation of this galaxy:
" Saw the Whale as a ghostly streak of light, almost like a short smoke trail of a passing shooting star. The galaxy's edges in places are so soft and subtle that its boundaries are difficult to determine. The galaxy’s core looks a tad brighter and elongated in an E-W direction as well. The dusty dark lane edge of the galaxy's arms is more distinct and obvious from the midpoint to the western edge. It definitely has a mottled appearance at high power. Also, a faint star (mag. 12) is noticeable near the northern edge of the galaxy’s glow."
Although not technically part of the this week's visual challenge, it is hard to view just the Whale and Calf without seeing NGC 4656/4657, the “Hockey Stick”, in the same field of view. Using a wide-field eyepiece, both pairs of galaxies can be squeezed into the field. NGC 4656 is another edge-on galaxy that has a companion. This companion, NGC 4657, interacts with NGC 4656 and distorts one end of the streak, giving observers the real impression of a hockey stick. They were discovered by William Herschel in 1787. This magnitude 10.4 galaxy and its companion is a nice addition to the observing experience.
Click on the finder chart below to help you locate this galaxy grouping among the constellations of the spring sky.
" Saw the Whale as a ghostly streak of light, almost like a short smoke trail of a passing shooting star. The galaxy's edges in places are so soft and subtle that its boundaries are difficult to determine. The galaxy’s core looks a tad brighter and elongated in an E-W direction as well. The dusty dark lane edge of the galaxy's arms is more distinct and obvious from the midpoint to the western edge. It definitely has a mottled appearance at high power. Also, a faint star (mag. 12) is noticeable near the northern edge of the galaxy’s glow."
Although not technically part of the this week's visual challenge, it is hard to view just the Whale and Calf without seeing NGC 4656/4657, the “Hockey Stick”, in the same field of view. Using a wide-field eyepiece, both pairs of galaxies can be squeezed into the field. NGC 4656 is another edge-on galaxy that has a companion. This companion, NGC 4657, interacts with NGC 4656 and distorts one end of the streak, giving observers the real impression of a hockey stick. They were discovered by William Herschel in 1787. This magnitude 10.4 galaxy and its companion is a nice addition to the observing experience.
Click on the finder chart below to help you locate this galaxy grouping among the constellations of the spring sky.
Leviathan's Deep Sky Object of the Week Archive:
Leviathan's Deep Sky Object of the Week #9:
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Leviathan's Deep Sky Object of the Week #10: Planetary Nebula -
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