Determining the age of a star can be one of the trickiest measurements an astronomer can make. Stars don’t obviate their age like humans do — with wrinkles around the eyes, grey hair, or brittle bones. Instead stars give hints at their age through things like their motion, rotation rate, chemistry, and placement in the Galaxy. Finding “extrema” on the age spectrum for stars — the oldest and youngest — is much easier than trying to find middle aged objects like our Sun. In this article, we reveal five fast facts about the youngest stars in the Galaxy.
Fast fact #1: When We Say “Young” We Might Mean Different Things
When we say “young” it can mean a variety of things. For some, “young” might mean an object that is less than one million years old, for others “young” might mean an object that is less than one billion years old. There is not a clear boundary that Astronomers use for youth. What’s more important is to understand the process by which stars are formed. Our best model of star formation begins with a giant molecular cloud of Hydrogen that collapses and fragments into pieces. The cores of those fragments accumulate mass and are called “protostars” before passing through a pre main sequence phase and finally contracting to a stable radius where they enter the stellar main sequence and nuclear fusion can proceed. The length of each of those stages varies depending on the mass of the star. Higher mass stars pass through each stage rapidly (< 1 Myr in some cases) whereas the lowest mass stars take magnitudes longer (< 500,000 years or so).
Fast fact #2: Young Stars Are Found In Groups
As stated above, stars are formed from the collapse of a giant molecular cloud of molecular Hydrogen. Rarely do we think that stars are formed in situ without a collection of family members nearby. When we talk about young stars we are usually referring to a collection of them. There are famous areas across the sky referred to as “star forming regions”. The most famous of all is probably the Orion star forming region which can be seen with the naked eye near the three bright stars in the belt of the constellation. This massive factory of star formation at just over 1300 light years away is the closest of its kind to the solar system. Based on our best diagnostics, the association appears to be <1 million years old so it would not have been present in the Mesozoic era when dinosaurs roamed the Earth. Aside from Orion there are many other giant star formation factories. There are what we call “complexes” that take up many degrees on the sky and consist of numerous substructures at different ages. The Scorpius-Centaurus association is one such complex. It is composed of the Upper Scorpius association, the Upper Centaurus Lupus association and the Lower Centaurus Crux association. Each is relatively close to the Sun at a distance of < 500 light years and the ages range from ~10 to ~15 million years. Hundreds of stars have been cataloged as part of the association and are studied in detail for their fundamental properties.
Fast fact #3: Young Stars Are “Active” And Can Be Very Violent
A fascinating aspect of young stars is how active and violent they can be. Important for understanding young stars is also knowing the difference between high mass and low mass stellar behavior. The most massive stars (several times the mass of the Sun) do not survive burning Hydrogen stably for very long. For instance, O and B stars which are the highest mass class of stars, are beacons for star forming regions because it is known that if they are around, they MUST be young since their main sequence lifetimes are just a few million years for O stars and < 100 million years for most B stars. These stars are much hotter than our own Sun and they give off a tremendous amount of radiation in the ultraviolet which ionizes the surrounding gas in the star forming area. These stars also have violent and highly energetic winds which bombard the surrounding lower mass stars. As can be clearly seen in the Orion star forming regions, young proto solar systems are evaporated by the winds of the higher mass stars. For the lower mass stars, youth means that they are also tremendously active with large flares of highly energetic particles emerging from their photospheres. Overall, the environment of a star forming region is one of large amounts of radiation and energy where gas and dust is illuminated and eventually destroyed by the fireworks of youth.
Fast fact #4: Young Stars Have Disks With Well Resolved Structure
In our current formulation of how young stars form, after the collapse of the giant molecular cloud and the formation of the protostar fragments, there is also a gaseous protoplanetary disk surrounding the young rotating star. The circumstellar disk of material is made of dense gas and dust which feeds the central core. It contains just a small percentage of the mass of the central star. The disk will transition over time but when the star is young, it is the proto environment for the formation of any planets, asteroids, comets, etc. which eventually define extrasolar planetary systems. Astronomers focus on imaging star forming regions to directly image and study the chemical composition of circumstellar disks and to try and catch young planetary systems in formation. The Atacama Large Millimeter Array – ALMA Radio Telescope has famously imaged many young stellar disks (e.g. TW Hydrae, HL Tauri, etc) and resolved gaps and rings at ages of just a few million years indicating that the planetary formation process might begin much earlier than previously suspected. Other instruments such as the Gemini Planet Imager (GPI), the Spectro-Polarimetric High-contrast Exoplanet Research instrument (SPHERE) have demonstrated how newly forming planets likely are sculpting and shaping these disks as they image them in the infrared.
The ESO image at the top of the page around the star cluster NGC 3572 shows how these clouds of gas and dust have been sculpted into whimsical bubbles, arcs and the odd features known as elephant trunks by the stellar winds flowing from this gathering of hot young stars. The brightest of these cluster stars are much heavier than the Sun and will end their short lives as supernova explosions.
Editor, Jackie Faherty, astrophysicist, Senior Scientist with AMNH. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. Her research team has won multiple grants from NASA, NSF, and the Heising Simons foundation to support projects focused on characterising planet-like objects. She has also co-founded the popular citizen science project entitled Backyard Worlds: Planet 9 which invites the general public to help scan the solar neighbourhood for previously missed cold worlds. A Google Scholar, Faherty has over 100 peer reviewed articles in astrophysical journals and has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognises scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments.