Physics Photo of the Week
May 17, 2013
The Whirlpool Nebula
The
Whirlpool Nebula, also known as Messier 51 (M51) is an
external galaxy. It is near the "tail" of the Big
Dipper, but it requires a telescope to see and long exposure
photography to record. This galaxy is approximately 25
million light years from Earth, is very popular for amateur
astronomers to photograph because it is rather "close"
compared to many galaxies that are hundreds of millions of
light years distant.
The distinct spiral structure is characteristic of many
distant galaxies including the Milky Way galaxy of which the
Sun and the Solar System are located. Galaxies such as
this are huge - about 100,000 light years across and contain
about 100 billion (1 x 1011) stars - most of the
stars are located in the "bright" cores of galaxies.
No individual stars are resolved in the photograph.
The stars scattered in the photo are "foreground" stars in
the Milky Way. The interesting spiral structure of
galaxies consists of clouds of "young" stars (stars younger
than about 20 million years). Much of the mass of
galaxies is dust and cold gas. As the galaxy slowly
rotates, the differential rotation causes the mass to bunch
up into shock fronts. The extra density in the shock
wave and the increased self gravity trigger star
formation. From studying the rotation rates of
galaxies, astronomers can determine the total mass of
galaxies. The total masses are about 20 times that
which is accounted for from the visible mass (visible
light-emitting stars as well as dust and gas clouds).
The missing dark mass - called "Dark Matter" - is an
important cosmological problem of 21st century
astronomy. Dark Matter has no interactions with
ordinary matter except for gravity which is very weak
compared with the other major forces in physics. At
the centers of all galaxies lies a super-massive black hole
- whose mass is millions of times larger than the mass of
the Sun. The black hole's presence is determined from
studying rotation rates of stars near the galaxy centers -
measured with dust-penetrating infrared radiation observed
by satellites for closer galaxies including the Milky Way.
A final feature of the Whirlpool Galaxy is that it
consists of two obvious galaxies. The smaller galaxy
has passed rather close to the big spiral and has distorted
the shape of the spiral arm that is closest to the smaller
galaxy. These galaxies may be involved in a cosmic
collision - a very slow process in terms of human lifetimes
because of the huge physical size of galaxies.
The photograph was made at the newly completed College
View Observatory using the 14 inch telescope donated
to Warren Wilson College by Gary Starkweather, WWC class of
1978. The camera is my own digital camera mounted at
the focal point of the telescope and recorded this past
Wednesday. I intend to obtain an astronomical grade
CCD camera which is considerably more sensitive and can
produce better photos.
This is the last Physics Photo of the Week for the
college academic year due to summer vacation. Physics
Photo of the Week will return when classes resume in late
August.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
Click here to see the Physics Photo of the Week Archive.
