Galaxy Spectra
Projects
 Basic
 Advanced
 - Hubble Diagram
 - Color
 - Spectral Types
 - H-R Diagram
 - Galaxies
 - Sky Surveys
 - Quasars
 - Image Processing
 Challenges
 For Kids
 Games and Contests
 Links to Others
Galaxies
 Classifying
 Hubble Tuning Fork
 Characteristics
 Clusters
 Separating
 Spectra
 Collisions
 Conclusion
 Your Results

Galaxy Spectra

When you look at the spectrum of a galaxy, you are really looking at the combination of spectra from the millions of stars in the galaxy. So studying the features of a galaxy spectrum tells you about the types of stars the galaxy contains, and the relative abundances of each type of star.

M51
Courtesy of
The Hubble Heritage Project

Galaxy spectra also clearly show you whether a galaxy contains star-forming regions called HII regions. HII is a spectral emission line that corresponds to ionized hydrogen - a hydrogen atom that has lost its electron. HII regions are areas of a galaxy where hydrogen nuclei and electrons are recombining to form neutral hydrogen.

When an electron recombines with a hydrogen nucleus, it loses energy and gives off a photon (you might recognize this as the opposite of what happens in absorption lines from the Spectral Types project). The HII spectral line is in the red part of the spectrum, so HII regions in galaxies have a beautiful red or pink tint in visible photographs (NOTE: SDSS uses the red filter for the green picture in a tri-color image. Therefore, HII regions do no appear red in SDSS images). You can clearly see HII regions in the photograph of M51 at the right.

Now, let's examine several galaxies both visually and spectroscopically.

Exercise 7: Below is a table of galaxies. Look up each galaxy by its Object ID using the Navigation Tool, then look up its spectrum by plate and fiber number using the Get Spectra Tool, then answer questions 7 through 10.

Click on the Object ID of each galaxy to launch the Navigation tool centered on the galaxy. Click the red Zoom button to bring the Zoom window to the front, then use the scrollbars to locate the green circle that marks the galaxy. Click on the green circle to see the galaxy's data.

Click on the Fiber number of each galaxy to see its spectrum in the Get Spectra tool.

Object ID

Plate Number

Fiber Number

2255031269261392

302

325

2255029906702376

277

290

2255030989357127

288

492

2255048446247171

283

565

1970862258258031

398

82

2255030731145307

300

592

1970729388998708

398

113

1970863334096941

400

608

2255030731931843

301

514

1970729661235319

403

422

2255029916729537

289

43

Launch the Navigation tool

Launch the Get Spectra tool

Question 7: What is the color of each galaxy? Color can be measured by u-r, with lower values being blue and higher values being red.

Question 8: What type of galaxy is each (you may need to set the Zoom window size to get a better view)?

Question 9: What strong emission or absorption lines do you see in each galaxy?

Question 10: Do you notice a relationship between color, emission/absorbtion lines, and galaxy type?

Galaxies do change as they age. Older galaxies have few young blue stars. Younger galaxies have lots of HII regions where stars are forming.