April 13, 2024

Hubble aims its powerful ultraviolet eye at superhot stars

Some stars are so massive and energetic that they are a million times brighter than the Sun. This type of star dominated the early Universe, playing a fundamental role in its development and evolution. The first of this type have already disappeared, but the modern Universe still forms stars of this type.

These hot, blue stars emit powerful ultraviolet energy that Hubble can detect from its position in low-Earth orbit.

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

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

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

“I believe the ULLYSES project will be transformative, impacting astrophysics in general – from exoplanets, to the effects of massive stars on the evolution of galaxies, to understanding the early stages of the evolving universe,” said Julia Roman-Duval, team leader of implementation of ULLYSES at the Space Telescope Science Institute (STScI) in Baltimore, Maryland. “In addition to the specific goals of the program, stellar data can also be used in the fields of astrophysics in ways we cannot yet imagine.”

ULYSSES spectra collected by Hubble can reveal the presence of chemical elements in stars. Image credit: Hubble/STScI/ULYSSES

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

Huge blue stars have powerful winds that shape their surroundings.  Hubble spectra can tell which way winds travel and how fast they travel.  The star represented by the teal line has slower winds than the star shown by the purple line.  Image credit: Hubble/STScI/ULYSSES
Huge blue stars have powerful winds that shape their surroundings. Hubble spectra can tell which way winds travel and how fast they travel. The star represented by the teal line has slower winds than the star shown by the purple line. Image credit: Hubble/STScI/ULYSSES

The spectra also reveal the metallicity of the 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 to hydrogen to date stars relative to the iron to hydrogen ratio of our own Sun.

These spectra show the iron content for two stars.  In this image, the star represented by the purple line has less iron, indicating that it is older than the other star.  Iron content affects a star's life and the strength of its winds.  Image credit: Hubble/STScI/ULYSSES
These spectra show the iron content for two stars. In this image, the star represented by the purple line has less iron, indicating that it is older than the other star. Iron content affects a star’s life and the strength of its winds. Image credit: Hubble/STScI/ULYSSES

In ULISSES, Hubble targeted hot blue stars in nearby galaxies with low metallicity, the kind that would have existed in the early Universe. At this point in the Universe’s life, they would contain nothing heavier than hydrogen and helium. This type of galaxy was common in the early universe. Only when these hot young stars died and scattered within themselves the elements they created would the heavier elements needed for rocky planets, water and even life be available. “The ULLYSES observations are a springboard for understanding these 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 of hot, massive stars: cool, red, low-mass, and faint stars. While the most massive stars form quickly, shine and die soon, these are the opposite. They take longer to form, are darker and last much longer. But they still emit winds and energy that shape their surroundings. They are called T-Tauri stars, stars so young they are still growing.

As part of the three-year ULYSSES survey, Hubble also observed cool, faint, low-mass stars, like the one in this artist's illustration, that are still growing by accreting material from their disks.  Image credit: Robert O'Connell (UVA), SOC-WFC3, ESO
As part of the three-year ULYSSES survey, Hubble also observed cool, faint, low-mass stars, like the one in this artist’s illustration, that are still growing by accreting material from their disks. Image credit: Robert O’Connell (UVA), SOC-WFC3, ESO

Despite their lower masses, these stars emit powerful radiation. During their formation, they are known to release powerful bursts of UV radiation and X-rays.

There are outstanding questions about T-Tauri stars and how they behave. Some of its processes are obscured. But Hubble spectra obtained from ULYSSES may 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 of these stars could eliminate the habitability of any planets that form around them.

ULYSSES data is not intended to answer any specific question. Instead, it is a huge database of detailed spectra that researchers can query to serve future research. The overall goal is to provide an in-depth database of spectra of young stars that are in their first 10 million years of life.

“A more complete understanding of the formation and lives of young stars has links to many other areas of astronomy, including the formation and evolution of galaxies, the mechanics and mass loss of supernovae, how the environments of stars impact planetary formation and how your emissions can play a role. in the composition of the interstellar medium, the gas and dust between the stars of a galaxy”, explains the ULYSSES website.

ULYSSES is an observation program developed by the research community for the research community. By extension, it also serves those of us who like to follow researchers discovering new things about the Universe.

“ULLYSES was originally conceived as an observation program using Hubble’s sensitive spectrographs. However, the program has been tremendously improved by coordinated, community-led ancillary observations with other ground- and space-based observatories,” said Roman-Duval. “This 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.”

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