How did scientists determine our location within the Milky Way galaxy–in other words, how do we know that our solar system is in the arm of a spiral galaxy, far from the galaxy’s center?

There is no short answer to this question, because astronomers have followed many lines of evidence to determine the location of the solar system in the Milky Way. But some of the general techniques can be outlined briefly.

Finding one’s location in a cloud of a hundred billion stars–when one can’t travel beyond one’s own planet–is like trying to map out the shape of a forest while tied to one of the trees. One gets a rough idea of the shape of the Milky Way galaxy by just looking around–a ragged, hazy band of light circles the sky. It is about 15 degrees wide, and stars are concentrated fairly evenly along the strip. That observation indicates that our Milky Way Galaxy is a flattened disk of stars, with us located somewhere near the plane of the disk. Were it not a flattened disk, it would look different. For instance, if it were a sphere of stars, we would see its glow all over the sky, not just in a narrow band. And if we were above or below the disk plane by a substantial amount, we would not see it split the sky in half–the glow of the Milky Way would be brighter on one side of the sky than on the other.

The position of the sun in the Milky Way can be further pinned down by measuring the distance to all the stars we can see. In the late 18th century, astronomer William Herschel tried to do this, concluding that the earth was in the center of a ‘grindstone’-shaped cloud of stars. But Herschel was not aware of the presence of small particles of interstellar dust, which obscure the light from the most distant stars in the Milky Way. We appeared to be in the center of the cloud because we could see no further in all directions. To a person tied to a tree in a foggy forest, it looks like the forest stretches equally away in all directions, wherever one is.

A major breakthrough in moving the earth from the center of the galaxy to a point about 3/5 away from the edge came in the early decades of this century, when Harlow Shapley measured the distance to the large clusters of stars called globular clusters. He found they were distributed in a spherical distribution about 100,000 light-years in diameter, centered on a location in the constellation Sagittarius. Shapley concluded (and other astronomers have since verified) that the center of the distribution of globular clusters is the center of the Milky Way as well, so our galaxy looks like a flat disk of stars embedded in a spherical cloud, or 'halo,’ of globular clusters.

In the past 75 years, astronomers have refined this picture, using a variety of techniques of radio, optical, infrared and even x-ray astronomy, to fill in the details: the location of spiral arms, clouds of gas and dust, concentrations of molecules and so on. The essential modern picture is that our solar system is located on the inner edge of a spiral arm, about 25,000 light-years from the center of the galaxy, which is in the direction of the constellation of Sagittarius.

Credit: Laurence A. Marschall in the department of physics at Gettysburg College


Why it’s important for scientists to study obscure things

For years, the kidney donation system had a heartbreaking problem: Many people are willing to donate a kidney to a loved one, but are incompatible due to blood type and other factors.

At the time, less than 20 people received kidneys from living donors. (Receiving a kidney from a living donor leads to much better outcomes for the patient.)

The solution to this problem —like many inventions and modern innovations— was built on decades of odd and obscure research.

When economists Gale and Shapley began their work on matchmaking, their research was mainly theoretical and abstract. But their insights provided the foundation for breakthroughs that had a real impact on people’s lives.

Read about their journey of discovery

It’s the BELATED  birthday of Adelaide Ames, who was born in 1900 in Rock Island, Illinois. Ames attended Radcliffe College, where in 1924 she became the first student to complete the college’s new graduate astronomy program. Unable to pursue her original career choice, journalism, Ames joined the research staff of the Harvard College Observatory. There, she and observatory director Harlow Shapley compiled what became known as the Shapley-Ames Catalog of Bright Galaxies. Published in 1932, the catalog contained 1249 objects and became a primary source for extragalactic astronomers for 60 years. Ames died that same year in a boating accident on Squam Lake, New Hampshire.

source: Physics Today


The Day Universe Grew a Billion Times Larger

On November 23, 1924, the New York Times carried a small story on the recent work by astronomer Edwin Hubble, following the recent work by Herbert Curtis and Harlow Shapley (known as the Great Debates) in 1920.  Additional work by Walter Baade showed individual stars were discernible within the Andromeda Galaxy, and  Ernst Öpik calculated the distance to Andromeda at approximately 1.5 million light years, far outside the calculated size of the Milky Way Galaxy.  The “Island Universe” theory proposed in the New York Times indicated that the Milky Way Galaxy was one galaxy among many, fundamentally changing the way astonomers (among others) understood the Universe.  While this debate had been evolving for several years among astronomers, this is the first popular account the receive widespread attention.  Astronomers now estimate that there are 170 billion galaxies in the universe, the most distant galaxy is  EGS-zs8-1 at 13.1 billion light years away, and just over 13 billion years old, forming only 670 million years after the Big Bang. 

The word galaxy has a long and complicated history to get to English. Arriving around the time of Geoffrey Chaucer in the late 14th century from Old French which in turn came from Late Latin to denote the Milky Way. The Latin form of galaxias was a direct transliteration of the Ancient Greek galaxias (adj.) from galaxias kyklos, literally a milky circle. The adjective galaxias came from the word for milk, gala (gen. galaktos). Our galaxy, which we call in English the Milky Way is a translation of Latin via lactea. From Chaucer’s House of Fame:

See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt.

Top image of the Andromeda Galaxy, bottom image EGS-zs8-1 both courtesy NASA/Hubble.