The Hyades Make up the Face of a Starry Bull

hyades star chart
A finder chart for the Hyades star cluster, typically visible from October to late March each year. Carolyn Collins Petersen

There's a starry bull in the sky called Taurus, the Bull that is visible from late October through March each year in the evening and pre-dawn skies. The face of the bull actually shows up in a vee-shaped star cluster in the sky that you can observe pretty easily. It's called the Hyades (pronounced "HIGH-uh-deez") and is a naked-eye object for most people. It's also visible to stargazers from almost anywhere on the planet. To find it, search out the constellation Taurus using a star chart or digital astronomy app

Thank the Ancients for Their Astute Observations

We owe our ancient stargazing ancestors a great deal when it comes to exploring fascinating things in the sky. For example, Greek astronomers identified the Hyades and its nearby neighbor — the Pleiades star cluster — thousands of years ago. Other cultures noted it, too, seeing everything from the face of a bull to the figures of gods and goddesses in the structure. There are star tales for just about every object in the sky, from every culture that has lived on our planet. The Hyades were thought to be the daughters of the god Atlas, and sisters to another group of daughters portrayed by the Pleiades. The Greeks weren't the only ones to tell tales involving these clusters. The Maori, for example, also told tales of the Hyades and Pleiades, as did cultures in ancient North America, China, and Japan. They were a popular sight and topic for mythology. 

The Stars of the Hyades

In reality, the Hyades are more closely related to another star cluster called "Praesepe", or the Beehive, which is an early spring object for Northern Hemisphere observers. Astronomers have long suspected that these two clusters had a common origin in an ancient cloud of gas and dust.

The Hyades stars lie about 150 light-years away from us and formed some 625 million years ago. They travel together through space in the same direction. Eventually, even though they do have a slight gravitational attraction for each other, they will go their separate ways, just as the Pleiades will do. At that point, even though their stars may have "unlinked" from the cluster, they're still traveling along the original trajectory. Astronomers call them "moving group" or a "moving cluster". 

There are about 400 stars in the Hyades, but we only see about 6 or 7 with the naked eye. The four brightest Hyades stars are red giants, types of stars that are aging. They've run through their nuclear fuel and are heading toward old age and eventual destruction. These stars are part of the V shape that ancient stargazers thought made up the face of a celestial bull named Taurus

Meet the Eye of the Bull: Aldebaran

The brightest star in the Hyades really isn't in the Hyades. It's called Aldebaran and its name was, like so many other star names, based in mythology. It happens to lie on the line of sight between us and the Hyades. It's an orange-hued giant that lies only 65 light-years away. Aldebaran is an old star that will eventually exhaust all its fuels and could eventually explode as a supernova before collapsing to form a neutron star or a black hole. Unlike Betelgeuse (the supergiant star in Orion's shoulder, which could explode anytime as a supernova), Aldebaran will likely be around for millions of years.

Both the Hyades and Pleiades are open clusters. There are many of these groupings of stars in the Milky Way and other galaxies. They are associations of stars born in the same clouds of gas and dust but are not tightly bound together by gravity as stars in globular clusters do. The Milky Way contains at least a thousand of these collections of stars and astronomers study them to understand how stars of similar ages evolve over time. From the time they form together in their birth clouds to the time they die, cluster members show us how stars of roughly the same age, but different masses, can change over time. Those changes are what lead to the astonishing diversity of stars in the universe. 

The highest-mass stars in the Hyades will use up their nuclear fuel very fast and die after a few hundreds of millions of years. Those same stars use up tremendous amounts of the original cloud as they form, which reduces the supply of star-making material available to their sibling stars. So, like the Hyades, many open star clusters contain members that are the same age, but some look older than others.