M51 is a spiral galaxy located approximately 23 million light years from Earth. It has two long, tightly wound spiral arms that are clearly defined. M51 is interacting with the smaller galaxy NGC 5195 located 25 million light years away. Using image analysis software, the diameter of M51 was measured to be approximately 63,708 light years and the diameter of one of its spiral arms was measured to be around 106,267 light years.

1. Classifying and Measuring
M51
Filipa Cruz

2. M51
Constellation: Canes Venatici
Right ascension: 13h 29m 52.7s
Declination: +47° 11′ 43″
Distance: 23 ± 4 Mly
Size (diameter): 60 000 ly
Apparent dimensions: 11′.2 × 6′.9
Apparent magnitude: 8.4
Notable features interacting with
NGC 5195, at the distance of 25 Mly.

3. M51 Classification
Hubble’s classification of galaxies divides regular galaxies into 3 broad classes:
ellipticals , lenticulars and spirals - based on their visual appearance.
According with the Hubble classification M51 is a spiral galaxie type Sa.

4.  Elliptical galaxy or a disc galaxy?
Disc galaxy.
 It is face-on to our line of sight or tilted?
It is face-on to our line of sight.
 Is barred?
It’s not a barred spiral.
 Number of spiral arms?
Two arms, long and tightly.
 Spiral arms are clearly defined, or vague and ragged in appearance?
The arms are clearly defined.
 Regions where there are a lot of bright, hot, young blue stars?
In the arms.
M51 Classification

5. With the program SalsaJ we can analyse and measure images.
In the M51 Image Info we can find the CCDSCALE: 0,3 arcsec/pixel
Analyse > Set Scale
Distance in pixels = 2008
Real distance = distance in pixels x CCDSCALE =
= 2008 x 0,3 = 602,4 arcsec
Image scale = 2008 / 602,4 = 3,3 pixels/arcsec
Measuring M51

6. To measure M51 diameter, we traced a line.
Analyse > Measure
Measuring M51
M51 diameter (two galaxies) = 554 arcsec
554 arcsec / 60 = 9,233 arcmin
To transform in light-years:
kly = arcmin x distance from Earth in Mly x 0,3 =
= 9,233 x 23 x 0,3 = 63,708 kly = 63 708 ly

7. M51 (spiral arm) = 924,042 arcsec
924,042 arcsec / 60 = 15,401 arcmin
To transform in light-years:
kly = arcmin x distance from Earth in Mly x 0,3 =
= 15,401 x 23 x 0,3 = 106,267 kly = 106 267 ly
Measuring M51 spiral arm

8. NGC 5195 diameter (small galaxy) = 354 arcsec
354 arcsec / 60 = 5,9 arcmin
To transform in light-years:
kly = arcmin x distance from Earth in Mly x 0,3 =
= 5,9 x 25 x 0,3 = 44,250 kly = 44 250 ly
Measuring NGC 5195

9. References
• http://resources.faulkes-telescope.com/course/view.php?id=64
• http://www.faulkestelescope.com/files/faulkestelescope.com/archive/
projects/galaxies/hubble_classification.pdf
• http://en.wikipedia.org/wiki/Hubble_sequence
• http://en.wikipedia.org/wiki/Whirlpool_Galaxy
• http://www.universetoday.com/35997/messier-51/
• http://messier.seds.org/m/m051.html