About this image: This near-infrared image from the ground-based VISTA VVV Survey shows the galactic bulge near Sagittarius A* (pronounced “A star”), the black hole at the Milky Way’s center. The region, outlined in white, shows five stacked fields of view from NASA’s Nancy Grace Roman Space Telescope that will be observed as part of its Galactic Bulge Time-Domain Survey, one of its three core community surveys. (Roman will also observe a sixth field at the galactic center that is not shown here.) Prior to Roman’s launch, a team of researchers sought to use Hubble to capture the same regions in preparation for potential microlensing events.
These events cause the light from a more distant object to warp as a mass precisely aligns in front of that object. These masses, therefore, act like lenses, bending the light from objects behind them like background stars. In this case, the glow from the densely packed stars within the galactic bulge would be the distant light source. Having these Hubble observations allows us to capture the moments before these microlensing events happen, providing astronomers a way to clearly characterize objects (stars, planets, and even stellar-mass black holes) that cause microlensing by passing in front of stars within the galactic bulge.
The colored lines representing the Hubble survey area are stylized and represent a large number of individual pointings.
Credits Image: NASA, Alyssa Pagan (STScI) - Acknowledgment: VISTA, Dante Minniti (UNAB), Ignacio Toledo (ALMA), Martin Kornmesser (ESO) //imag
A follow-up observation by NASA’s Hubble Space Telescope shows a field containing a microlensing event that was captured by the Optical Gravitational Lensing Experiment (OGLE) in 2013. This provides an example of how a Hubble image could be used to analyze future microlensing events spotted by NASA’s Nancy Grace Roman Space Telescope.
In gravitational microlensing, the gravity of a foreground object acts as a lens, magnifying and distorting the light of a background star when the two objects align in the sky. Credits Image: NASA, ESA, Sean Terry (UMD), Jay Anderson (STScI) - Image Processing: Alyssa Pagan (STScI)
This graphic illustrates a microlensing event, which occurs when the light from a distant object warps as a mass, such as a star (depicted here) or a stellar-mass black hole, precisely aligns in front of that object. In this image, a red, foreground star intervenes between the telescope, acting as the “lens,” bending, and magnifying the light of the yellow background star. Unlike some gravitational lensing events, which occur at the scale of galaxies or galaxy clusters, microlensing events occur on a much smaller scale, such as that of individual stars. The lensing effect is, therefore, much smaller.
This image also provides a representation of what the background star would look like to a telescope in a microlensing event. Because of the curvature of space around the background star (represented by the white arrows that curve around it in the image), the background star appears to increase in brightness as the event begins before decreasing in apparent brightness as it falls out of alignment. The graph at bottom plots the apparent brightness of the background star over time. Credits Illustration: NASA, STScI, Joyce Kang (STScI)
This video shows a zoom into the Milky Way’s galactic bulge near the galactic center. As it zooms in, the view changes from the near-infrared 2MASS survey to the VISTA VVV survey (both ground-based). At the conclusion of the zoom, part of the region of the galactic bulge that will be surveyed by Roman’s Galactic Bulge Time-Domain Survey is highlighted with five stacked fields of view. (Roman will also observe a sixth field at the galactic center that is not shown here.)
Prior to Roman’s launch, a team of researchers are using NASA’s Hubble Space Telescope to observe the same regions to enable better analysis of microlensing events detected by Roman. The colored lines representing the Hubble survey area are stylized and represent a large number of individual pointings. The video also labels Sagittarius A* (pronounced “A star”), the black hole at the Milky Way’s center.
Credits Video: NASA, Alyssa Pagan (STScI) - Acknowledgment: VISTA, Caltech, Caltech/IPAC, Sean Terry (UMD), Jay Anderson (STScI), Dante Minniti (UNAB), Ignacio Toledo (ALMA), Martin Kornmesser (ESO), 2MASS
The Milky Way’s
galactic bulge, the bulbous region that surrounds the galactic center, contains a dense collection of stars, planets, and other free-floating objects. This region has been studied for decades with numerous ground-based and space-based telescopes, including NASA’s Hubble and James Webb space telescopes. Soon, NASA’s Nancy Grace Roman Space Telescope will be the first to make studying the galactic bulge a part of its core science objectives, building on the data collected from all observatories before it. Roman’s field of view will cover more area at a far faster cadence than previous space telescopes, allowing it to survey millions of stars and find thousands of new exoplanets.
To support Roman in characterizing numerous stars and planets, astronomers sought to use Hubble to observe many of the same areas of the galactic bulge that Roman will observe in its core
Galactic Bulge Time-Domain Survey. By comparing Hubble data taken months or years earlier to new Roman data, astronomers will be better able to interpret Roman’s forthcoming observations. The Roman telescope team is targeting as soon as early September 2026 for launch.
“A top priority of our Hubble survey is to cover as much sky area as possible,” said Sean Terry, project lead and assistant research scientist from the University of Maryland, College Park and NASA’s Goddard Space Flight Center in Greenbelt.
A paper about the team’s work published May 11, 2026 in the
Astrophysical Journal.
‘Small’ lenses, large discoveries
Many planetary systems within the Milky Way evolve much like our solar system did, beginning with the collapse of a cosmic gas cloud, the growth of a star, and the formation of surrounding planets. However, in some systems, different events can result in a planet being ejected from the system where it formed. Hundreds of these “rogue planets” will be detected by Roman’s Galactic Bulge Time-Domain Survey, in addition to previously unseen,
isolated neutron stars, and even black holes with masses similar to our Sun.
This survey consists of six 72-day observing seasons during which Roman will take a snapshot every 12 minutes of a large portion of the
bulge (approximately 1.7 square degrees of the region, or the area of 8.5 full moons). While it will detect a variety of targets, the survey
is optimized to look for a specific type of event known as microlensing.
Microlensing events, a type of gravitational lensing event, occur when the light from a more distant object is warped by the mass of a closer object along the line of sight. These events occur on a much smaller scale than larger lensing events (on the order of individual stars instead of galaxies or galaxy clusters) and allow us to search for exoplanets between us and the densely packed stars within the galactic bulge.
“The great thing about microlensing is that we’ll be able to do a complete census of objects as small as Mars that are moving between us and these fields in the bulge, no matter what it is,” said co-author Jay Anderson of the Space Telescope Science Institute in Baltimore.
When a telescope observes a lensing object, such as a bright star, aligning with a star in the galactic bulge, it can be difficult for astronomers to decipher which of the two the starlight comes from. Therefore, timing is a key consideration. If astronomers can identify light sources separately before a microlensing event occurs, it becomes far easier to disentangle them.
To collect this pre-Roman data, astronomers used the Hubble Space Telescope to conduct a large-scale survey, which began in the spring of
2025, covering much of the same area that Roman will observe in the Galactic Bulge Time-Domain Survey. The size of this program is even
larger than two previous surveys (each around 0.5 square degrees) that led to
Hubble’s largest mosaic, that of our neighboring Andromeda galaxy, which took over 10 years to assemble.
“The main goal of these observations is to be able to identify objects that participate in lensing events during the Roman survey,
catching them before they undergo the lensing event,” said Anderson. “When, in a couple of years, an event happens during Roman's long stare
at the field, we can go back and say, ‘This was a red star, this was a blue star, and the event happened when the red star went in front of the
blue star.’”
The data from Hubble also will help shape the analysis of the lensing objects themselves. The microlensing event itself measures only a ratio
of the masses of a host star and its planet. With data from stars before or after their microlensing events, however, scientists would be
able to measure the stars’ individual masses, echoing the way Hubble
previously determined the mass of a star and its planet in the Milky Way. This method turns a more opaque measurement of the relationship between a star and its planet into one far more certain.
“Instead of estimating a mass ratio of a planet that's orbiting a star, we can say that we're confident it's a Saturn-mass planet orbiting
a star that's 0.8 solar masses, for example,” Terry said. “So with the help of precursor imaging from Hubble you can hope to get direct
measurements of the masses as opposed to indirect mass ratios.”
While exoplanet discovery is a large part of Roman’s Galactic Bulge Time-Domain Survey, observing such a large area with Hubble also can help identify areas of extinction, dense pockets of dust and gas that absorb or scatter light, allowing us to create maps detailing where we can see stars and where we can’t.
Hubble’s survey also has provided the crucial beginning of a brand-new catalog of stars, which will help astronomers characterize the
host stars of exoplanets discovered by Roman. The research team predicts Roman will add to Hubble’s star catalog by an order of
magnitude.
“This Hubble survey will build a catalog of 20 to 30 million point sources,” said Terry. “But, by the end of the Galactic Bulge Time-Domain
Survey, Roman may measure about 200 to 300 million, and it will produce, essentially, some of the deepest images ever taken of any part
of the sky.”
The data from the most recent Hubble survey is available in the
Mikulski Archive for Space Telescopes.
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our
fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA Goddard manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
The Nancy Grace Roman Space Telescope is managed at NASA Goddard with participation by NASA's Jet Propulsion Laboratory in Southern
California; Caltech/IPAC in Pasadena, California; the Space Telescope Science Institute; and a science team comprising scientists from various
research institutions. The primary industrial partners are BAE Systems, Inc. in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida;
and Teledyne Scientific & Imaging in Thousand Oaks, California.
About This Release
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Media Contact:
Matthew Brown
Space Telescope Science Institute, Baltimore
Christine Pulliam
Space Telescope Science Institute, Baltimore
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