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 a very powerful close by stellar island to our Milky Manner — 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 exterior our personal Milky Manner galaxy — at a distance of roughly 2.5 million light-years or roughly 25 Milky Manner diameters. Previous to that, astronomers had lengthy thought that the Milky manner 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 probably the most complete survey of this engaging empire of stars. The Hubble telescope is yielding new clues to the evolutionary historical past of Andromeda, and it appears markedly totally different from the Milky Manner’s historical past.
With out Andromeda as a proxy for spiral galaxies within the universe at giant, astronomers would know a lot much less in regards to the construction and evolution of our personal Milky Manner. That is as a result of we’re embedded contained in the Milky Manner. That is like attempting to grasp the format of New York Metropolis by standing in the course of Central Park.
“With Hubble we are able to get into huge element about what’s taking place on a holistic scale throughout the whole disk of the galaxy. You’ll be able to’t try 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 seem like grains of sand throughout the seaside. However that is simply the tip of the iceberg. Andromeda’s whole inhabitants is estimated to be 1 trillion stars, with many much less large stars falling beneath Hubble’s sensitivity restrict.
Photographing Andromeda was a herculean process 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 complete mosaic was carried out beneath two Hubble packages. In whole, 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 have been obtained at near-ultraviolet, seen, and near-infrared wavelengths utilizing the Superior Digicam for Surveys and the Broad Subject Digicam 3 aboard Hubble to {photograph} the northern half of Andromeda.
This program was adopted up by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), lately printed in The Astrophysical Journal and led by Zhuo Chen on the College of Washington, which added photos 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 packages 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 packages present details about the age, heavy-element abundance, and stellar plenty inside Andromeda. It will enable astronomers to differentiate 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 ‘Prepare Wreck’
Although the Milky Manner and Andromeda shaped presumably across the identical time many billions of years in the past, observational proof reveals that they’ve very totally 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 energetic latest star-formation and interplay historical past than the Milky Manner.
“Andromeda’s a practice wreck. It appears prefer it has been by way of some form of occasion that prompted it to kind a whole lot of stars after which simply shut down,” mentioned Daniel Weisz on the College of California, Berkeley. “This was most likely because of 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 which will have interacted with Andromeda previously. Pc simulations counsel that when an in depth encounter with one other galaxy makes use of up all of the obtainable interstellar fuel, star formation subsides.
“Andromeda appears like a transitional kind of galaxy that is between a star-forming spiral and a form of elliptical galaxy dominated by getting old purple stars,” mentioned Weisz. “We are able to inform it is obtained this large central bulge of older stars and a star-forming disk that is not as energetic as you would possibly anticipate given the galaxy’s mass.”
“This detailed take 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 photos in a single publicity. These observations will complement and lengthen Hubble’s large dataset.
