Introduction to Science in the Galactic Plane

Lesson 4: Angular Size and Distance

Our Galaxy with annotations


This exercise will allow the instructor/teacher to present to his/her students a lesson on measuring the angular size of NGC 7027; one of the objects observed in the First Galactic Plane Survey. In the next exercise, the temperature of NGC 7027 will be calculated based on your results of this exercise. This activity is usable for high school physical science, astronomy, and physics classes.


The angular size of Milky Way objects can be measured using simple trigonometric relationships. By measuring the angular size of a source, you will be able to calculate its distance from the earth. Distance measurements allow you to better understand your place in the Milky Way galaxy. The angular size is also used in a correction factor to calculate the temperature of a source in the next exercise.


The coordinates that you measured for each object in Sky Image Processor will be converted into astronomical coordinates of RIGHT ASCENSION and DECLINATION using Aladin. Now that you have RIGHT ASCENSION and DECLINATION coordinates, the angular size will be measured using the Aladin website:


To get the most out of this activity, students should have a basic understanding of the following concepts:
(a) loading image processing software
(b) loading required images
(c) mathematical analysis which includes angle measurement


Recommendation: students work in groups of NO MORE than 3/group; therefore, students are more individualy involved in the research.
(1) Import Aladin software.
(2) Choose: Load
(3) Choose: original FITS; this choice is necessary to display FITS images.
(4) Server Selection; select Aladin if importing FITS image from the Aladin website, or local if importing FITS image from your machine.
Specify a target object by either astronomical coordinates (if known) or by object name; for example, NGC7027. SUBMIT.
Pick an image from survey choices; for example, 14.1' X 14.1' full resolution image. SUBMIT.

(1) To change color or reverse image, from right toolbar select histogram. Color choice or reversed image may improve image analysis.
(2) From right toolbar, choose the distance tool. Place cursor on left edge of NGC 7027 and drag cursor horizontally across image. Read size below image. This is the 'time' size of image. To convert to angular size, multiply time size by 15. The result is the angular size of NGC 7027 in arcseconds. Record this value
(3) To calculate distance to source, use: d = 57400/(angular size)

NGC 7027 is a planetary nebula located about 3000 light years from us in the constellation Cygnus. A planetary nebula is the shell of gas ejected from a dying star. The outer material of the star has been lifted off and expands at speeds of 20 to 30 km/s. A typical planetary nebula has a diameter of 1 light year. How do your results of angular size and distance compare with a typical planetary nebula?

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Galactic Plane Science