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Starts With A Bang —

Fossilized Relic Discovered By Hubble Is A Bridge To The Milky Way’s Past

A “double burst” of star formation in a globular cluster may explain our galaxy’s mysterious center.

“It matters not what someone is born, but what they grow to be.” 
–J.K. Rowling

When new stars form in the Universe, they always form out of whatever gas is available.

The giant star-forming region 30 doradus in the gas-rich Tarantula nebula. Image credit: ESO/P. Crowther/C.J. Evans.

New stars form from large, collapsing gas clumps early on, with many forming globular clusters in galactic halos.

This wide-field image, based on data from Digitized Sky Survey 2, shows the whole region around the stellar grouping Terzan 5. Image credit: ESO/Digitized Sky Survey 2.

Globular clusters contain stars numbering from tens-of-thousands to tens-of-millions, all within a few hundred light years.

The ancient globular cluster Messier 15, a typical example of one of these objects. Image credit: ESA/Hubble & NASA.

Most globulars formed when the Universe was young, with stars over 12 or even 13 billion years old.

A map of the nearest globular clusters to the Milky Way’s center. Image credit: William E. Harris, McMaster U., and Larry McNish.

The Milky Way alone contains around 200 globulars, including Terzan 5.

Unlike most globulars, Terzan 5 contains two different populations of stars.

Many of the redder, fainter stars shown here belong to the older population of stars in Terzan 5. Image credit: NASA/ESA/Hubble/F. Ferraro.

One is a normal, metal-poor population of 12 billion years, but the second formed 7 billion years later!

The younger, brighter, bluer stars are so numerous that they point to a second burst of star-formation just 4.5 billion years ago. Image credit: NASA/ESA/Hubble/F. Ferraro.

That second burst of star formation required a surprising amount of neutral gas: at least 100,000,000 Suns’ worth.

The less dense regions of stars in Terzan 5 really highlight the differences, spectroscopically, between the two populations. Image credit: NASA/ESA/Hubble/F. Ferraro.

These dual-populations with different metal-richnesses are similar to our galactic bulge’s stars.

An infrared view of the stars in the Milky Way’s central bulge. Image credit: 2MASS (Two-micron all-sky survey).

Gaseous clumps persist for aeons in the galactic bulge, but none had ever remained for so long in globular clusters.

Previously discovered globular clusters with “gas” in them, like this one, are likely simply globulars obscured by matter that lies along the line-of-sight in the Milky Way, not belonging to the globular itself. Image credit: Image credit: NASA/JPL-Caltech/Univ. of Wyoming/DSS.

Yet this must be the case to create these dual populations.

The central core of Terzan 5 highlighted, with its vast mix of stellar types and ages. Image credit: NASA/ESA/Hubble/F. Ferraro.

Terzan 5 is likely one of the bulge’s primordial building blocks.

The globular cluster Terzan 5 as seen by the ESO’s Very Large Telescope, with other data as well. Image credit: ESO-VLT, F.R. Ferraro et al., HST-NICMOS, ESA/Hubble & NASA.

This relic links us to the Milky Way’s past.

Mostly Mute Monday tells the story of a single astronomical phenomenon or object primarily in visuals, with no more than 200 words of text.

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