"Long before it's in the papers"
January 28, 2015


Our galaxy no longer “little sister”

Jan. 6, 2009
Courtesy National Radio Astronomy Observatory
and World Science staff

Fas­ten your seat belts: our gal­axy is faster, heav­i­er, and more likely to col­lide than we thought, re­search­ers say.

As­tro­no­mers mak­ing new mea­sure­ments of the Milky Way say our home gal­axy is spin­ning about 100,000 miles per hour faster than pre­vi­ously un­der­stood.

That find­ing re­quires in­creas­ing the gal­ax­y’s es­ti­mat­ed weight by 50 per­cent, said Mark Reid of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass., bring­ing its weight even with the neigh­bor­ing An­drom­e­da Gal­axy. 

“No long­er will we think of the Milky Way as the lit­tle sis­ter of the An­drom­e­da Gal­axy in our Lo­cal Group fam­i­ly,” he re­marked.

The larg­er mass, in turn, im­plies a great­er gravita­t­ional pull that in­creases the like­li­hood of col­li­sions with the An­drom­e­da gal­axy or smaller near­by ga­lax­ies. Some re­search­ers have spec­u­lat­ed that such a col­li­sion could toss our So­lar Sys­tem in­to An­drom­e­da.

Our So­lar Sys­tem is estimated to be 28,000 light-years from the Milky Way’s cen­ter (a light-year is the distance light travels in a year.) At that mea­sured dis­tance, the new ob­serva­t­ions in­di­cate, we’re mov­ing at about 600,000 miles per hour in our Ga­lac­tic or­bit, up from the pre­vi­ous es­ti­mate of 500,000 miles per hour, Reid and col­leagues said.

The sci­en­tists are us­ing the U.S. Na­tional Sci­ence Founda­t­ion’s Very Long Base­line Ar­ray ra­dio tel­e­scope to re­make the map of the Milky Way. The team is con­duct­ing a long-term pro­gram to meas­ure dis­tances and mo­tions in our Gal­axy. They re­ported their re­sults at the Amer­i­can As­tronomical So­ci­ety’s meet­ing in Long Beach, Ca­lif. on Jan. 5. 

“These mea­sure­ments use the tra­di­tion­al sur­vey­or’s meth­od of tri­an­gula­t­ion and do not de­pend on any as­sump­tions based on oth­er prop­er­ties, such as bright­ness, un­like ear­li­er stud­ies,” said Karl Menten of the Max Planck In­sti­tute for Ra­dio As­tronomy in Bonn, a mem­ber of the team.

Reid and his col­leagues found oth­er sur­prises, too. Meas­ur­ing the dis­tances to mul­ti­ple re­gions in a sin­gle spir­al arm al­lowed them to cal­cu­late the an­gle of the arm. “These mea­sure­ments,” Reid said, “in­di­cate that our Gal­axy probably has four, not two, spir­al arms of gas and dust that are form­ing stars.” 

Re­cent sur­veys by NASA’s Spitzer Space Tel­e­scope sug­gest that old­er stars re­side mostly in two spir­al arms, rais­ing a ques­tion of why the old­er stars don’t ap­pear in all the arms. An­swer­ing that ques­tion, the as­tro­no­mers say, will re­quire more mea­sure­ments and a deepe­r un­der­stand­ing of how the gal­axy works.

* * *

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Fasten your seat belts: our galaxy is faster, heavier, and more likely to collide than we thought, researchers say. Astronomers making new measurements of the Milky Way say our home galaxy is spinning about 100,000 miles per hour faster than previously understood. That finding requires increasing the galaxy’s estimated weight by 50 percent, said Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., bringing its weight even with the neighboring Andromeda Galaxy. “No longer will we think of the Milky Way as the little sister of the Andromeda Galaxy in our Local Group family,” he remarked. The larger mass, in turn, implies a greater gravitational pull that increases the likelihood of collisions with the Andromeda galaxy or smaller nearby galaxies. Some researchers have speculated that such a collision could toss our Solar System into Andromeda. Our Solar System is about 28,000 light-years from the Milky Way’s center. At that distance, the new observations indicate, we’re moving at about 600,000 miles per hour in our Galactic orbit, up from the previous estimate of 500,000 miles per hour, Reid and colleagues said. The scientists are using the National Science Foundation’s Very Long Baseline Array radio telescope to remake the map of the Milky Way. The team is conducting a long-term program to measure distances and motions in our Galaxy. They reported their results at the American Astronomical Society’s meeting in Long Beach, Calif. on Jan. 5. “These measurements use the traditional surveyor’s method of triangulation and do not depend on any assumptions based on other properties, such as brightness, unlike earlier studies,” said Karl Menten of the Max Planck Institute for Radio Astronomy in Bonn, a member of the team. Reid and his colleagues found other surprises, too. Measuring the distances to multiple regions in a single spiral arm allowed them to calculate the angle of the arm. “These measurements,” Reid said, “indicate that our Galaxy probably has four, not two, spiral arms of gas and dust that are forming stars.” Recent surveys by NASA’s Spitzer Space Telescope suggest that older stars reside mostly in two spiral arms, raising a question of why the older stars don’t appear in all the arms. Answering that question, the astronomers say, will require more measurements and a deeper understanding of how the galaxy works.