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ARP 273

(James Clark)
(James Clark - Close-up)

Information...

ARP-273 is an interacting pair of galaxies, 300 million light years away in the constellation of Andromeda.

The 'ARP' catalogue number refers to the 'Atlas of Peculiar Galaxies'. The larger of the two is also referenced as UGC1810 / PGC8961, and the smaller as UGC1813 / PGC8970.

Capturing these very deep 'Deep Sky' images is quite a challenge, so hat's off to James for this distant effort. In the image you can see that the graviational attraction between the two is tidally distorting the spiral shape of UGC1810. The interaction is also known to be creating active star formation at UGC1813's nucleus.

For more info see the Wikipedia entry.

 

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Map

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Measuring Angles

Hold your arm at full length, then close one eye & use the hand shapes shown above to measure the angular distance between the stars.

(Ain't anatomy wonderful!)

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Apparent Magnitude

The apparent magnitude of a star is a measure of how bright it appears from Earth. The scale was introduced over 2,000 years ago by the Greek astronomer Hipparchus, who grouped stars into six categories. The brightest 20 or so were deemed to be 'first magnitude', slightly dimmer stars 'second magnitude', and so on until the barely visible stars were classed as 'sixth magnitude'.

Later it was recognised that our eyesight, once it has been given time to get used to darkness, has a logarithmic response. i.e. a Mag. 1 star is actually 2.512 times brighter than a Mag. 2 star, or 6.310 times brighter than a Mag. 3 star (2.512 x 2.512 = 6.310).

The six Magnitudes thus corresponds to a 2.5126 difference in brightness or 100x.

Apparent magnitude

Today the scale has now been extended, so that brighter objects can have an apparent magnitude of 0 or even negative. The brightest star Sirius, for example, has an apparent magnitude of -1.44 and the Sun is a whopping -26.74, due to it's close proximity to Earth.

 

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