Gorgeous New Galactic ‘Growth Chart’

SCIENCE

Astronomers have measured the age of 70,000 stars across the Milky Way and put the results into the world’s largest-ever “age map” of our galaxy. (BBC)

Use our hi-res map to dig deeper into the Milky Way.

Teachers, scroll down for a quick list of key resources in our Teachers’ Toolkit.

This 3-D video embeds new data in a model of a Milky Way-like galaxy. The youngest stars are shown in blue, middle-aged stars in green, and the oldest stars in red.

Discussion Ideas

The dazzling new data from the good folks at the Max Planck Institute for Astronomy tracks the ages of 70,000 stars in the Milky Way. Is that the number of stars in our galaxy?
- Not even close! According to NASA, “there isn’t a way to simply count the number of stars in the Milky Way individually,” but estimates range from about 100 billion to more than 400 billion.

So the survey is a stellar sample, not a stellar census. What type of stars did astronomers sample?
- Astronomers determined the ages and plotted the geography of red giant stars. Red giants have huge surface areas, low surface temperatures, and a vaguely reddish color. (The sun will be a red giant someday.)
  - Red giants “are very bright and we see them at very large distances,” which makes them perfect for this sort of research. According to the Max Planck study, “red giant luminosity depends very little on the star’s age or its chemical composition. Hence, a study of red giants does not run the risk of being biased towards stars of certain ages or compositions—it has a good chance of providing a fair sample.”

This graph shows age distribution for a sample of red giant stars ranging from the galactic center to the outskirts of the Milky Way. The horizontal axis shows distances from the galactic center, while the vertical axis shows distances above or below the galactic plane (not to scale). Distances are provided in kpc, or kiloparsecs—a kpc is about 3,262 light-years. Color-coding shows the median age of the red giants observed at that particular position. Age is given in gyr, or gigayears—a gyr is one billion years. Image by M.Ness and the Max Planck Institute for Astronomy — This graph shows age distribution for a sample of red giant stars ranging from the galactic center to the outskirts of the Milky Way. The horizontal axis shows distances from the galactic center, while the vertical axis shows distances above or below the galactic plane (not to scale). Distances are provided in kpc, or kiloparsecs—a kpc is about 3,262 light-years. Color-coding shows the median age of the red giants observed at that particular position. Age is given in gyr, or gigayears—a gyr is one billion years.
Image by M.Ness and the Max Planck Institute for Astronomy

The new map shows the distribution of younger and older stars. What does “younger” and “older” mean in the context of our galaxy? Take a look at the video or graph above for some help.
- The youngest stars are less than a billion years old, while the oldest are more than 12 billion years old. (The Sun, by the way, is comfortably middle-aged at about 4.6 billion years old.)

This graph displays the relationship between the abundances of certain chemical elements in stellar atmospheres and the mass of the star. The horizontal axis compares the abundance of metals (M) (in astronomy, metals are all elements heavier than hydrogen) to hydrogen (H). The vertical axis compares the abundance of carbon (C) to nitrogen (N). Color-coding shows the median age of stars in each region. Image by M.Ness and the Max Planck Institute for Astronomy — This graph displays the relationship between the abundances of certain chemical elements in stellar atmospheres and the mass of the star. The horizontal axis compares the abundance of metals (M) (in astronomy, metals are all elements heavier than hydrogen) to hydrogen (H). The vertical axis compares the abundance of carbon (C) to nitrogen (N). Color-coding shows the median age of stars in each region.
Image by M.Ness and the Max Planck Institute for Astronomy

The new galactic growth map had two major research components: the APOGEE survey and the Kepler Asteroseismic Science Consortium: APOKASC. The first part of the research involved the APOGEE survey. What is APOGEE?
- The Apache Point Observatory Galaxy Evolution Experiment (APOGEE) is one part of a massive redshift survey using a telescope at the Apache Point Observatory in the Land of Enchantment.
  - Redshift surveys allow astronomers to calculate the velocity of stellar objects (in this case, red giants). Redshifts tell astronomers how quickly, and in what direction, stellar objects are moving away from them.
  - Redshift surveys also allow astronomers to calculate the chemical composition of stellar objects using spectroscopy. Spectroscopy is the science of the measurement of light that is reflected, absorbed, or emitted by different materials. In the graph above, astronomers used spectroscopy to determine the relative abundance of hydrogen, carbon, and nitrogen.

The second part of the research involved the Kepler satellite. What is Kepler?
- Kepler is a space observatory with a mission to detect Earth-like planets orbiting distant stars. Learn more about Kepler here.
  - The Kepler mission involves the spacecraft analyzing a specific region of space for a long time, allowing astronomers to determine a stellar object’s mass. “If we know the mass of these Kepler stars, we can determine their ages,” explained lead researcher Melissa Ness.

Astronomers were able to build a model relating a star’s mass and age (determined by Kepler data) to its spectrum (determined by APOGEE data). Red for old, blue for new, green for in between!
Image by M.Ness and the Max Planck Institute for Astronomy

What did the “age map” display? How has our galaxy grown up?
- Just as astronomers suspected, “Our galaxy started out as a small disc, and it’s grown from the inside out.”
  - The oldest stars are nearer to what is now the center of the galaxy and the central galactic bulge. Younger stars rim the spiral edges. Here’s where we are.

TEACHERS’ TOOLKIT

BBC: Largest ever ‘age map’ traces galactic history

Nat Geo: 2002 Milky Way Galaxy hi-resolution map

Space.com: Red Giant Stars: Facts, Definition & the Future of the Sun

SDSS: APOGEE: Probing the Evolution of the Milky Way

NASA: Kepler: The Search for Habitable Planets

(extra credit!) Max Planck Institute for Astronomy: In-depth information: Making (galactic) history with big data: First global age map of the Milky Way