The Hubble Aims Its Powerful Ultraviolet Eye at Super-Hot Stars

Some stars are so massive and so energetic that they’re a million times brighter than the Sun. This type of star dominated the early Universe, playing a key role in its development and evolution. The first of its kind are all gone now, but the modern Universe still forms stars of this type.

These hot, blue stars emit powerful ultraviolet energy that the Hubble can detect from its perch in Low-Earth Orbit.

In December 2023, astronomers completed a three-year survey of these hot stars. It’s one of the Hubble’s largest and most ambitious surveys. It’s called ULLYSES (Ultraviolet Legacy Library of Young Stars as Essential Standards), and in it, astronomers gathered detailed information on almost 500 stars.

UV emissions from hot young stars provide a window into some of the processes inside these stars. UV can’t be observed from Earth because the ozone layer blocks it. That’s one of the reasons the Hubble was built. From its perch, it can gather high-resolution UV images. That’s the impetus for ULLYSES.

The survey doesn’t contain images of all the stars. Instead, the Hubble gathered spectra from 220 stars and combined them with Hubble archival data on 275 additional stars. Powerful ground-based telescopes also made a contribution, though not in UV. The result is a very rich dataset consisting of detailed spectra from both hot, bright, massive stars and from cool, dim, low-mass stars.

“I believe the ULLYSES project will be transformative, impacting overall astrophysics – from exoplanets to the effects of massive stars on galaxy evolution, to understanding the earliest stages of the evolving universe,” said Julia Roman-Duval, Implementation Team Lead for ULLYSES at the Space Telescope Science Institute (STScI) in Baltimore, Maryland. “Aside from the specific goals of the program, the stellar data can also be used in fields of astrophysics in ways we can’t yet imagine.”

Spectra can tell astronomers more than just the metallicity of the stars. They can also reveal the powerful stellar winds coming from the hot blue stars.

Spectra also reveal the metallicity of stars. Stars with lower metallicity are typically older than stars with higher metallicity. A critical part of stellar metallicity concerns the iron content. Astronomers use iron content and its ratio with hydrogen to date stars in relation to our own Sun’s iron and hydrogen ratio.

In ULYSSES, Hubble targeted hot blue stars in nearby galaxies with low metallicity, the type that would’ve existed in the early Universe. At that point in the Universe’s life, they would’ve contained nothing heavier than hydrogen and helium. This type of galaxy was common in the very early universe. Only once these hot young stars died and spread the elements they created inside themselves would the heavier elements needed for rocky planets, water, and even life be available. “ULLYSES observations are a stepping stone to understanding those first stars and their winds in the Universe and how they impact the evolution of their young host galaxy,” said Roman-Duval.

ULLYSES also observed stellar counterparts to the massive, hot stars: cool, red, low-mass, and dim stars. While the more massive stars form quickly, burn bright, and die soon, these ones are the opposite. They take longer to form, are dimmer, and last much longer. But they still emit winds and energy that shape their surroundings. They’re called T-Tauri stars, stars so young they’re still growing.

Despite their lower masses, these stars emit powerful radiation. During their formation, they’re known to unleash powerful blasts of both UV and X-ray radiation.

There are outstanding questions about T-Tauri stars and how they behave. Some of their processes are obscured. But the Hubble spectra from ULYSSES can provide some answers. They can reveal how much energy T-Tauri stars release as they grow and how powerful their winds are. Their powerful winds can alter their protoplanetary disks, blowing material away and making it unavailable for planet formation. In some cases, the powerful energy from these stars could eliminate the habitability of any planets forming around them.

The ULYSSES data is not meant to answer any specific question. Rather, it’s a massive database of detailed spectra that researchers can query to serve future research. The overarching goal is to provide an in-depth database of spectra from young stars that are in the first 10 million years of their lives.

“More fully understanding the formation and lives of young stars has connections to many other areas in astronomy, including galaxy formation and evolution, the mechanics and mass loss of supernovas, how stars’ environments impact planet formation, and how their emissions may play a role in the makeup of the interstellar medium, the gas and dust between stars in a galaxy,” the ULYSSES website explains. 

ULYSSES is an observing program designed by the research community for the research community. By extension, it also serves those of us who like to follow along as researchers discover new things about the Universe.

“ULLYSES was originally conceived as an observing program utilizing Hubble’s sensitive spectrographs. However, the program was tremendously enhanced by community-led coordinated and ancillary observations with other ground- and space-based observatories,” said Roman-Duval. “Such broad coverage allows astronomers to investigate the lives of stars in unprecedented detail and paint a more comprehensive picture of the properties of these stars and how they impact their environment.”