Why the Milky Way May Be Facing a Midlife Crisis   
   By Nola Taylor Redd, SPACE.com Contributor   
   Space.com | SPACE.com - 10 hrs ago   
       
   Our Milky Way galaxy and its neighbor, the Andromeda galaxy, seem to be going   
   through a midlife crisis.   
       
   New research reveals that both galaxies are in the middle of transitioning   
   from young, star-forming regions into older, stagnant ones, a transition that   
   is revealed by the galaxies' color. Generally, such a change comes after two   
   galaxies collide, astronomers said, but this pair seems to be making the shift   
   on its own.   
       
   In galaxies, star formation rates and color are closely related. But, analyses   
   of the shade of the Milky Way are surprisingly rare.   
   A team of Australian astronomers, led by Simon Mutch, of the Swinburne   
   University of Technology in Melbourne, sought to determine the color the Milky   
   Way and the nearby spiral Andromeda galaxy. [Stunning Photos of Galaxy   
   Collisions]   
       
   Too young to feel this old   
       
   Instead of the young, active signs they expected, the astronomers determined   
   that both galaxies are not the typical blue or red color, but instead, fall in   
   an in-between state of green.   
       
   Scientists have long regarded the Milky Way as a "typical" spiral galaxy, but   
   in regards to color, the galaxy doesn't quite follow the standard model.   
   "Green galaxies are commonly thought to represent galaxies which are   
   undergoing the transition from being young, dynamic, energetic, star-forming   
   blue galaxies to being old, lethargic red galaxies," Mutch told SPACE.com in   
   an email interview.   
       
   "In terms of a human being, this transitional phase could be thought of being   
   akin to a midlife crisis."   
       
   Astronomers can't figure out the color of the Milky Way easily by examining it   
   from the inside, because the position of the sun within the galaxy makes it a   
   challenge.   
       
   "Determining the state of our own galaxy, while we're stuck inside it, is very   
   difficult to do," Mutch said. "The phrase 'hard to see the forest for the   
   trees' rings true here. When we look at the galaxy, much of it is obscured   
   from our view by intervening gas and dust."   
       
   Viewing the Milky Way from the outside   
       
   To peek inside, Mutch and his team studied the mass of stars, the rate of star   
   formation and the brightness and color of the stars within the two galaxies.   
   These measurements provide a snapshot of the galactic pair.   
   The astronomers then modeled how galaxies grow, selecting an evolutionary   
   track that fulfilled several of the properties of the Milky Way and Andromeda   
   galaxies today.   
       
   Now, instead of looking at the galaxy from the inside, the team was able to   
   get a glimpse from the outside instead, which gave them a clear view of the   
   color.   
       
   Star formation dominates in blue galaxies, where the glow of young stars gives   
   a galaxy its shine. As the stars die, they explode in supernova blasts that   
   distribute gas throughout the galaxy, which is then recycled into newborn   
   stars.   
       
   But galaxies aren't sitting quietly; they are constantly moving in the   
   expanding universe. When galaxies collide, gas flows into the black hole at   
   the newly merged center.   
       
   The resulting active galactic nucleus, or AGN, is among the brightest radio   
   signals in the universe, and can be seen across great distances. Because it   
   consumes the gas that baby stars need, the AGN slows star formation, and the   
   galaxy loses its blue hue and gradually shifts to red, the astronomers said.   
   What caused the midlife crisis?   
       
   But the Milky Way and the Andromeda galaxy are shifting from blue to red   
   without a collision, which was a surprising discovery, the researchers   
   explained.   
       
   Since they are already slowing down, when they crash into each other in the   
   future, the collision most likely won't generate a new powerhouse.   
       
   "Our finding that both the Milky Way and Andromeda are green suggests that   
   there will be little cold gas left in both these galaxies when they merge   
   sometime in the next 5 billion years or so," Mutch said. "They will likely be   
   unable to produce an AGN."   
       
   So, why is the Milky Way running out of dust? Mutch isn't sure.   
       
   Sometimes black holes will inject large amounts of energy into the surrounding   
   region, Mutch explained. This keeps new gas from being accreted.   
       
   "However, we know from observations that the central black hole of our galaxy   
   is not particularly active," Mutch said.   
       
   The new color "provides us with an interesting open question as to what   
   exactly is causing the Milky Way and Andromeda to be running out of fuel for   
   producing new stars."   
       
   Details of the study were published in the July edition of the Astrophysical   
   Journal.   
       
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   Roger   
      
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