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New star system discovered orbiting our Milky Way galaxy

A team of astronomers have made an astonishing discovery — UMa3/U1 — the faintest star system ever observed orbiting our galaxy, the Milky Way.

Dubbed Ursa Major III / UNIONS 1 (UMa3/U1), it’s an incredibly dim and ancient group of stars located 30,000 light-years away in the constellation Ursa Major (which contains the Big Dipper).

UMa3/U1 hiding in plain sight

According to the research team from Yale University and the University of Victoria, UMa3/U1 was invisible for so long because it’s incredibly faint and small. We’re talking a mere 60 stars spanning just about 10 light-years across.

For comparison, a single light-year is nearly 6 trillion miles. Even with powerful telescopes, it would be like trying to spot a handful of dust motes floating by a spotlight.

Despite its small size, this little cosmic neighbor is actually quite close, a mere 30,000 light-years from Earth. It resides in the constellation Ursa Major (which contains the Big Dipper).

Is UMa3/U1 a galaxy or a star cluster?

The key question astronomers have is this: Is UMa3/U1 a true dwarf galaxy, or is it a star cluster? The answer might come down to a mysterious, invisible substance – dark matter.

Galaxies are thought to be held together by the gravitational pull of dark matter – a type of matter we can’t see directly but that scientists know exists due to its gravitational effects.

On the other hand, gravity alone usually binds together the stars in star clusters, often without the assistance of dark matter.

Yet, the surprising spread of UMa3/U1’s stars hasn’t led to their disintegration by the Milky Way’s gravitational forces. Could the dark matter be the unseen glue holding this cosmic relic together?

“The object is so puny that its long-term survival is very surprising,” explains Will Cerny, a Yale University graduate student involved in the study. “One might have expected the harsh tidal forces from the Milky Way’s disk to have ripped the system apart by now, leaving no observable remnant.”

UMa3/U1 as a galaxy

Firstly, UMa3/U1 might be a genuine dwarf galaxy, an entity with an incredibly low amount of visible matter compared to what we typically observe in such galaxies

This characteristic makes it an intriguing subject of study, as it suggests that UMa3/U1 could be predominantly composed of dark matter. 

If UMa3/U1 is indeed a dwarf galaxy rich in dark matter, it could provide invaluable insights into the role of dark matter in galaxy formation and evolution. 

It could support the theory that many such dark matter-dominated galaxies exist but remain hidden from our view, potentially revolutionizing our understanding of the universe’s structure.

Star cluster

Alternatively, UMa3/U1 could be a star cluster on the brink of disintegration. This perspective portrays UMa3/U1 as a cosmic anomaly, a cluster of stars that has stayed bound together for billions of years and is now possibly in its final stages of disintegration due to the Milky Way’s gravitational forces.

Observing such a disintegration in real-time would offer a unique opportunity to study the life cycle of star clusters and the dynamic processes involved in their evolution and dissolution.

If scenario one turns out to be true, it would be thrilling evidence supporting our current leading theory of how the universe works – the Lambda Cold Dark Matter (LCDM) model.

This model suggests that when our galaxy formed, it pulled in hundreds of smaller satellite systems that should still orbit it today.

UMa3/U1 cosmic ghost hunting

“Whether future observations confirm or reject that this system contains a large amount of dark matter, we’re very excited by the possibility that this object could be the tip of the iceberg — that it could be the first example of a new class of extremely faint stellar systems that have eluded detection until now,” says Cerny.

The team used powerful telescopes in Hawai’i, like the W. M. Keck Observatory and the Canada-France-Hawai’i Telescope (CFHT), to zero in on this celestial mystery.

Now, more observations are needed to reveal the true nature of UMa3/U1.

More about our Milky Way galaxy

Our home, the Milky Way, is a majestic spiral galaxy spanning over 100,000 light-years across. It contains an estimated 200 to 400 billion stars, including our own Sun.

Structure and composition

The Milky Way boasts a distinct structure, with spiral arms extending from a central bulge. These arms, named Perseus, Sagittarius, Centaurus, and Cygnus, contain a mix of young, hot stars and older, cooler ones. The galaxy’s disk also houses vast clouds of gas and dust, serving as the birthplaces for new stars.

At the heart of the Milky Way lies a supermassive black hole named Sagittarius A*. This cosmic behemoth, with a mass of over 4 million Suns, exerts a powerful gravitational influence on the surrounding stars and gas.

Astronomers study this region intensely to better understand the nature of black holes and their role in galaxy evolution.

Our solar system’s neighborhood

The Sun resides in a relatively quiet corner of the Milky Way, about 27,000 light-years from the galactic center. The solar system is part of a local stellar neighborhood called the Local Bubble, a region of space characterized by a lower density of gas and dust compared to other parts of the galaxy.

The Milky Way contains a diverse array of stars, ranging from ancient, metal-poor stars in the halo to younger, metal-rich stars in the disk. Astronomers classify these stars into different populations based on their age, chemical composition, and location within the galaxy. Studying these populations helps us understand the galaxy’s formation and evolution.

The Milky Way is not alone in the cosmic void. It is part of the Local Group, a collection of over 50 galaxies bound together by gravity.

The Andromeda galaxy, our largest galactic neighbor, is on a collision course with the Milky Way. In about 4.5 billion years, these two galaxies will merge, reshaping the cosmic landscape.

Fascination of the unknown

“This discovery may challenge our understanding of galaxy formation and perhaps even the definition of a ‘galaxy’,” explains Simon Smith, an astronomy graduate student at the University of Victoria and lead researcher on the study.

Whether a dwarf galaxy or a star cluster, UMa3/U1 reminds us of the vast secrets hiding in the cosmic darkness. And the excitement of discovery? Well, that shines as bright as ever.

The study is published in The Astrophysical Journal and


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