Hubble team members reflect on the completion of one of the telescope’s most ambitious observation programs and the impact it’s already having in astronomy.
About This Article
Since June 2020, the Hubble Space Telescope has observed a representative sample of young stars—those recently formed and those still in the process—as part of its three-year ULLYSES (Ultraviolet Legacy Library of Young Stars as Essential Standards) program. Completed at the end of 2023, this large Hubble program has built a rich ultraviolet dataset that will support astronomers now and long into the future. Three members of the ULLYSES implementation team, Implementation Team Lead Julia Roman-Duval, Technical Implementation Lead Will Fischer, and Data Products Lead Jo Taylor, explain why this program is so critical for future astronomical research.
How does a program like ULLYSES, the largest Hubble program to date, take shape?
Julia Roman-Duval: The program was initiated by STScI’s director at the time, Ken Sembach. By then, Hubble had completed several other large programs, including its Deep Fields and Frontier Fields. Sembach convened meetings with the science staff to discuss what would be good ideas for large programs. Looking at massive stars in the nearby universe to complement the gaps in our archive was suggested and brought to the Space Telescope Users Committee and the astronomical community. A working group was assembled with experts from outside the institute to assess the possibilities of a program focused on ultraviolet spectroscopy of young stars. The committee drafted an extensive report outlining its science goals and observing strategy. Once that report was finalized, the Hubble mission office started assembling a team to implement the program.
What are some of the big questions we have about young stars that ULLYSES data can help answer?
Roman-Duval: It’s important to emphasize that Hubble has delivered a legacy dataset of young stars in ultraviolet light. Ultraviolet light can only be observed from space, and Hubble is the only active telescope that can observe ultraviolet light in high resolution. This dataset is made up of spectra, which encode information about each star’s temperature, chemical composition, and rotation.
The first component of ULLYSES is the study of high-mass young stars. A big question we have about high-mass stars is: How do their winds evolve and change when their abundance of heavy elements decreases? These winds impact not only the immediate environment of the massive stars, but galaxies as a whole, for example by blowing gas and dust into their halos. A lot of diagnostics for stars’ winds are in ultraviolet light, and only Hubble can capture those spectra.
Will Fischer: The second component of the program is focused on low-mass stars that are still actively forming. These stars are about as massive as the Sun, down to about a quarter of the mass of the Sun. The mass of a star is its single most important characteristic—it determines how it’s going to evolve. These low-mass stars are surrounded by disks of gas and dust. They are pulling in mass from those disks, where planets may also be forming. With ULLYSES, we’re trying to learn how this process works. How does matter make its way from the disk onto the star and increase the star’s mass over time? How a star gathers mass also affects how fast the star rotates, which is important for the star’s evolution.
When matter from the disk crashes into the star, it releases a lot of ultraviolet and X-ray light. Right now, Hubble is the only telescope with high resolution ultraviolet spectrograph capabilities. It’s also giving us nearly simultaneous visible and near-infrared coverage. These combined observations are important because young low-mass stars pull in mass at rates that vary over short timescales.
With these extensive studies of young low- and high-mass stars, Hubble is helping us obtain a full picture of star formation in the nearby universe, where we can resolve individual stars. This will help astronomers understand star formation in the more distant universe, where individual stars cannot be resolved.
Why does ULLYSES include data from other missions?
Roman-Duval: Data from existing missions complement Hubble’s dataset. NASA’s FUSE (Far Ultraviolet Spectroscopic Explorer) observed far-ultraviolet light, and took spectra of pretty faint stars—even by Hubble’s standards. We combined FUSE and Hubble data to get full ultraviolet coverage and additional diagnostics for massive stars. In addition, a program with the ground-based Very Large Telescope extended coverage of some massive stars into visible and near-infrared light. By combining all these observations, the massive stars and their winds can be fully characterized.
Jo Taylor: We essentially stitched the FUSE data onto the lower end of the wavelength regime from Hubble. We actually have a couple of FUSE experts at the institute who guided our data calibration, and even repaired some of the FUSE products that had calibration issues.
Fischer: For the low-mass stars, we’re adding the Very Large Telescope’s XSHOOTER spectrograph data, which extends coverage into near-infrared light. For the low-mass stars, as you go to longer wavelengths, you’re basically moving farther from the star and into the inner parts of the disk that surround it. These near-infrared data allow you to track the temperature of the disk and its composition.
We also relied on Las Cumbres Observatory, a robotic global telescope network, to frequently image the stars before and after Hubble took its observations. The robotic telescopes also took images of each star every 15 minutes while Hubble was also observing that same star, which helps put all the ULLYSES spectroscopy into context.
What do you think we’ll learn thanks to ULLYSES? How will the data be used?
Fischer: As a result of ULLYSES, I’m seeing a revitalization of work by people who study low-mass stars. My dissertation advisor earned her Ph.D. in 1980 and at the time there were maybe a dozen people around the world who cared about these stars. Now, we have ongoing meetings to discuss ULLYSES’ low-mass stars with five times as many people. Hubble’s program is already inspiring a younger generation of researchers and restarting scientific conversations.
Taylor: Everything is disseminated through our archive, the Mikulski Archive for Space Telescopes (MAST), which is a very big platform. We worked with our colleagues to create a search form for ULLYSES data. Having everything in one place allows people to easily retrieve data for these stars and enhances the legacy of the project.
Roman-Duval: The code behind our data products is open source. Users can tweak our parameters and make a product to their liking. It’s a great tool that goes beyond the ULLYSES data. It’s already been adapted and applied elsewhere.
Three years. Almost 1,000 orbits. Does a particular memory stick out to you during your time supporting the ULLYSES program?
Roman-Duval: Some of the community members told us, “Thank you so much for doing this, because it’s extremely helpful to us,” and shared the science they are doing. I find that extremely rewarding. Every time we go to the Space Telescope Users Committee or conferences, we get good feedback. I also think the first and last observations were memorable, and when we released some of the first targets. I think there were about 14 million views.
Taylor: Releasing time-series data for the four low-mass stars was a highlight for me. We took data over a two-week period, and then would go back a year later and take two more weeks of data. As you can imagine, a lot of data accumulated from that alone, but we eventually added other Hubble archival data. Now we have many years’ worth of spectra for four targets. Putting all of that together and looking at the plots, you can see how the stars change. It’s just incredible to see that variability visualized. It’s mesmerizing.
What’s next? The final ULLYSES dataset was released on December 12, 2023, and the team will continue to organize opportunities to engage with this comprehensive data collection collaboratively. In March 2024, the team will host a workshop at the Space Telescope Science Institute to celebrate the beginning of a new era for research about young stars.
Hubble's 2023 Accomplishments
The full release of the ULLYSES data is only one of the Hubble mission’s many 2023 accomplishments. Teams continued offering new ways to request time-sensitive observations, most notably through the introduction of a new Target of Opportunity class called “Flexible Thursday.” Scientists may request to observe transient astronomical events with Hubble one Thursday per month. The past year also marked the first year of cooperation between Hubble and the James Webb Space Telescope, providing a glimpse of the exciting data that can result from what will be an ongoing partnership.
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Top portion of an infographic titled “ULLYSES: Hubble’s Ultraviolet Survey of Young Stars.”
Introduction text: The Hubble Space Telescope surveyed the actively forming and recently formed stars in ultraviolet light over three years. The result is ULLYSES, the Ultraviolet Legacy Library of Young Stars as Essential Standards, an unmatched set of information known as spectra that will help researchers pin down how young stars form and shape their nearby environments.
First section
The topmost portion of the graphic shows the Hubble Space Telescope at the center. The telescope is pointing toward a target slightly to the right. The mirror cover, mirror opening, base of the cylindrical body, and surfaces of the solar panels to the left of the body are visible. Beneath the telescope is a thin rainbow that takes the shape of a rectangle, beginning with purple and ending in red. The rainbow extends upward to meet the base of the Hubble Space Telescope body.
In the background to the left is half of a large blue star with active areas appearing brighter, and a haze of blue extending toward Hubble. To the right is a portion of the Cosmic Reef, a Hubble image. A giant red nebula, filled with transparent and opaque gas and dust appears as a semi-circle that is open to the right. A few bright blue stars are at the center, along with brilliant blue regions at center and toward the top.
Three text areas appear near Hubble, the first appearing at top left and pointing to the telescope. Text: Why was Hubble tapped for this role? Only Hubble has the space-based location and instruments to capture spectra in ultraviolet light, making ULLYSES a data set of lasting importance.
On the box below this, a line at the top points to the blue star. A line at the bottom of the box points to the rainbow at the center. The text reads: Ultraviolet light traces some of the hottest material and the most energetic processes of stars.
The third box, pointing to a blue portion in the Cosmic Reef, at right reads: Hubble observed stars that are actively forming or recently formed.
Second section
Three boxes fill the width. Within them, from left to right:
141, number of low-mass stars in the library. This box includes an orange star, which takes up about one sixth of the box.
355 number of high-mass stars in the library. This box includes a blue star, which is roughly twice the size of the star to the left.
63 days, amount of time Hubble spent building this library.