Hubble Looked as Far Back in Time as it Could, and Still Couldn’t See the First Generation of Stars in the Universe

Astronomers don’t know exactly when the first stars formed in the Universe because they haven’t been observed yet. And now, new observations from the Hubble Space Telescope suggest the first stars and galaxies may have formed even earlier than previously estimated.

Why? We *still* haven’t seen them, even with the best telescope we’ve got, pushed to its limits.

A group of researchers used Hubble to look back in time (and space) as far as it could see, hoping to study these first generation of stars of the early Universe, which are called Population III stars. Hubble peered and squinted back to when the Universe was just 500 million years old – which is thought to be Hubble’s limit — and found no evidence of these very first stars.

The name — Population III – is a little confusing. Shouldn’t these first stars be called Population I stars? Let’s face it, astronomers have never been great about naming things. The name Population I had already been taken when astronomers classified the stars of the Milky Way as Population I (stars like the Sun, which are rich in heavier elements).  Then, the name Population II was used to classify older stars in the Milky Way with a low heavy-element content.

And that left the name Population III to classify the stars that were forged from the primordial material that emerged from the Big Bang, approximately 13.8 billion years ago. Population III stars must have been made solely out of hydrogen, helium and lithium, the only elements that existed before processes in the cores of these stars could create heavier elements, such as oxygen, nitrogen, carbon and iron.

This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACS J0416. This is one of six clusters that was studied by the Hubble Frontier Fields programme, which yielded the deepest images of gravitational lensing ever made. Scientists used intracluster light (visible in blue) to study the distribution of dark matter within the cluster. Credit:NASA, ESA, and M. Montes (University of New South Wales, Sydney, Australia)

 Rachana Bhatawdekar of the European Space Agency led this most recent study, probing the early Universe from about 500 million to 1 billion years after the Big Bang. They studied the star cluster MACSJ0416 (see earlier comment about astronomers naming nomenclature) and the surrounding field with the Hubble Space Telescope, along with using supporting data from NASA’s Spitzer Space Telescope and the ground-based Very Large Telescope of the European Southern Observatory).

These observations were part of Hubble’s Frontier Fields program, which observed six distant galaxy clusters from 2012 to 2017, and produced the deepest observations ever made of galaxy clusters and the  galaxies located behind them. This was achieved by using the gravitational lensing effect, where the masses of foreground galaxy clusters are large enough to bend and magnify the light from the more distant objects behind them.  This allows Hubble to use these cosmic magnifying glasses to study objects that are beyond its nominal operational capabilities.

These observations revealed galaxies between 10 to 100 times fainter than any previously observed.

Illustration of the depth by which Hubble imaged galaxies in prior Deep Field initiatives, in units of the Age of the Universe. The goal of the Frontier Fields is to peer back further than the Hubble Ultra Deep Field and get a wealth of images of galaxies as they existed in the first several hundred million years after the Big Bang. Note that the unit of time is not linear in this illustration. Illustration Credit: NASA and A. Feild (STScI)

Bhatawdekar and her team developed a new technique that removes the light from the bright foreground galaxies that constitute these gravitational lenses. This allowed them to discover galaxies with lower masses than ever previously observed with Hubble, at a distance corresponding to when the Universe was less than a billion years old.

“We found no evidence of these first-generation Population III stars in this cosmic time interval,” said Bhatawdekar. “These results have profound astrophysical consequences as they show that galaxies must have formed much earlier than we thought.”

Since these observations are at the limits of Hubble, it puts one more task on the to-do list for the upcoming James Webb Space Telescope.

More info:
HST press release
Hubblecast 118: How the first stars transformed the Universe
UT article: Astronomers are hoping to see the very first stars and galaxies in the Universe