Within the years following the launch of NASA’s Hubble Area Telescope, astronomers have tallied over 1 trillion galaxies within the universe. However just one galaxy stands out as crucial close by stellar island to our Milky Means — the magnificent Andromeda galaxy (Messier 31). It may be seen with the bare eye on a really clear autumn night time as a faint cigar-shaped object roughly the obvious angular diameter of our Moon.
A century in the past, Edwin Hubble first established that this so-called “spiral nebula” was really very far outdoors our personal Milky Means galaxy — at a distance of roughly 2.5 million light-years or roughly 25 Milky Means diameters. Previous to that, astronomers had lengthy thought that the Milky method encompassed the whole universe. In a single day, Hubble’s discovery turned cosmology the other way up by unveiling an infinitely grander universe.
Now, a century later, the house telescope named for Hubble has completed essentially the most complete survey of this attractive empire of stars. The Hubble telescope is yielding new clues to the evolutionary historical past of Andromeda, and it seems markedly completely different from the Milky Means’s historical past.
With out Andromeda as a proxy for spiral galaxies within the universe at giant, astronomers would know a lot much less concerning the construction and evolution of our personal Milky Means. That is as a result of we’re embedded contained in the Milky Means. That is like making an attempt to grasp the format of New York Metropolis by standing in the midst of Central Park.
“With Hubble we will get into monumental element about what’s taking place on a holistic scale throughout the whole disk of the galaxy. You may’t do this with every other giant galaxy,” mentioned principal investigator Ben Williams of the College of Washington. Hubble’s sharp imaging capabilities can resolve greater than 200 million stars within the Andromeda galaxy, detecting solely stars brighter than our Solar. They appear to be grains of sand throughout the seashore. However that is simply the tip of the iceberg. Andromeda’s complete inhabitants is estimated to be 1 trillion stars, with many much less huge stars falling under Hubble’s sensitivity restrict.
Photographing Andromeda was a herculean activity as a result of the galaxy is a a lot larger goal on the sky than the galaxies Hubble routinely observes, which are sometimes billions of light-years away. The total mosaic was carried out beneath two Hubble applications. In complete, it required over 1,000 Hubble orbits, spanning greater than a decade.
This panorama began with the Panchromatic Hubble Andromeda Treasury (PHAT) program a few decade in the past. Photos had been obtained at near-ultraviolet, seen, and near-infrared wavelengths utilizing the Superior Digicam for Surveys and the Large Discipline Digicam 3 aboard Hubble to {photograph} the northern half of Andromeda.
This program was adopted up by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), not too long ago revealed in The Astrophysical Journal and led by Zhuo Chen on the College of Washington, which added photographs of roughly 100 million stars within the southern half of Andromeda. This area is structurally distinctive and extra delicate to the galaxy’s merger historical past than the northern disk mapped by the PHAT survey.
The mixed applications collectively cowl the whole disk of Andromeda, which is seen nearly edge-on — tilted by 77 levels relative to Earth’s view. The galaxy is so giant that the mosaic is assembled from roughly 600 separate fields of view. The mosaic picture is made up of at the very least 2.5 billion pixels.
The complementary Hubble survey applications present details about the age, heavy-element abundance, and stellar plenty inside Andromeda. This may enable astronomers to tell apart between competing situations the place Andromeda merged with a number of galaxies. Hubble’s detailed measurements constrain fashions of Andromeda’s merger historical past and disk evolution.
A Galactic ‘Practice Wreck’
Although the Milky Means and Andromeda shaped presumably across the similar time many billions of years in the past, observational proof reveals that they’ve very completely different evolutionary histories, regardless of rising up in the identical cosmological neighborhood. Andromeda appears to be extra extremely populated with youthful stars and weird options like coherent streams of stars, say researchers. This suggests it has a extra lively latest star-formation and interplay historical past than the Milky Means.
“Andromeda’s a practice wreck. It seems prefer it has been by way of some form of occasion that brought about it to kind a number of stars after which simply shut down,” mentioned Daniel Weisz on the College of California, Berkeley. “This was in all probability resulting from a collision with one other galaxy within the neighborhood.”
A attainable wrongdoer is the compact satellite tv for pc galaxy Messier 32, which resembles the stripped-down core of a once-spiral galaxy that will have interacted with Andromeda previously. Laptop simulations recommend that when a detailed encounter with one other galaxy makes use of up all of the accessible interstellar gasoline, star formation subsides.
“Andromeda seems like a transitional kind of galaxy that is between a star-forming spiral and a kind of elliptical galaxy dominated by getting older crimson stars,” mentioned Weisz. “We are able to inform it is acquired this large central bulge of older stars and a star-forming disk that is not as lively as you would possibly anticipate given the galaxy’s mass.”
“This detailed have a look at the resolved stars will assist us to piece collectively the galaxy’s previous merger and interplay historical past,” added Williams.
Hubble’s new findings will assist future observations by NASA’s James Webb Area Telescope and the upcoming Nancy Grace Roman Area Telescope. Basically a wide-angle model of Hubble (with the identical sized mirror), Roman will seize the equal of at the very least 100 high-resolution Hubble photographs in a single publicity. These observations will complement and prolong Hubble’s big dataset.
