Pleione is a binary star system in the Pleiades star cluster, within the constellation of Taurus. It has the variable star designation BU Tauri (BU Tau) and the Flamsteed designation 28 Tauri (28 Tau). Pleione is located close on the sky to the brighter star Atlas, so is difficult for stargazers to distinguish with the naked eye despite being a fifth magnitude star.

The brighter star of the Pleione binary pair, component A, is a hot type B star 184 times more luminous than the Sun. It is classified as Be star with certain distinguishing traits: periodic phase changes and a complex circumstellar environment composed of two gaseous disks at different angles to each other. The primary star rotates rapidly, close to its breakup velocity, even faster than Achernar. Although some research on the companion star has been performed, stellar characteristics of the orbiting B component are not well known.

28 Tauri is the star's Flamsteed designation and BU Tauri its variable star designation. The name Pleione originates with Greek mythology; she is the mother of seven daughters known as the Pleiades. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Pleione for this star. It is now so entered in the IAU Catalog of Star Names.

With an apparent magnitude of +5.05 in V, the star is rather difficult to make out with the naked eye, especially since its close neighbour Atlas is 3.7 times brighter and located less than 5 arcminutes away. Beginning in October of each year, Pleione along with the rest of the cluster can be seen rising in the east in the early morning before dawn. To see it after sunset, one will need to wait until December. By mid-February, the star is visible to virtually every inhabited region of the globe, with only those south of 66° unable to see it. Even in cities like Cape Town, South Africa, at the tip of the African continent, the star rises almost 32° above the horizon. Due to its declination of roughly +24°, Pleione is circumpolar in the Northern Hemisphere at latitudes greater than 66° North. Once late April arrives, the cluster can be spotted briefly in the deepening twilight of the western horizon, soon to disappear with the other setting stars.

Pleione apparently became significantly dimmer sometime between the 17th and 19th centuries. In 1937, William A. Calder discovered that the star's brightness was still varying by a small amount (~1/6 magnitude). Pleione is now classified as a Gamma Cassiopeiae type variable star, with brightness fluctuations that range between a 4.8 and 5.5 visual magnitude. It has a spectral classification of B8Vne, a hot main sequence star with "nebulous" absorption lines due to its rapid rotation and emission lines from the surrounding circumstellar disks formed of material being ejected from the star.

There has been significant debate as to the star's actual distance from Earth. The debate revolves around the different methodologies to measure distance—parallax being the most central, but photometric and spectroscopic observations yielding valuable insights as well. Before the Hipparcos mission, the estimated distance for the Pleiades star cluster was around 135 parsecs or 440 light years. When the Hipparcos Catalogue was published in 1997, the new parallax measurement indicated a much closer distance of about 119±1.0 pc (388±3.2 ly), triggering substantial controversy among astronomers. The Hipparcos new reduction produced a broadly similar distance of 120±2 pc. If the Hipparcos estimate were accurate, some astronomers contend, then stars in the cluster would have to be fainter than Sun-like stars—a notion that would challenge some of the fundamental precepts of stellar structure. Interferometric measurements taken in 2004 by the Hubble Telescope's Fine Guidance Sensors and corroborated by studies from Caltech and NASA's Jet Propulsion Laboratory showed the original estimate of 135 pc or 440 ly to be the correct figure. The Gaia EDR3 parallax is 7.24±0.1255 mas, indicating a distance around 138 pc. This is relatively imprecise for a Gaia result due to the brightness of the star, but still with a statistical margin of error similar to the Hipparcos results.

In 1942 Otto Struve, one of the early researchers of Be Stars, stated that Pleione is "the most interesting member of the Pleiades cluster". Like many of the stars in the cluster, Pleione is a blue-white B-type main sequence star with a temperature of about 11,000 K. It has a bolometric luminosity of 184 L_☉ assuming a distance of roughly 130 pc. With a radius of 3.7 R_☉ and mass that is 2.9 M_☉, Pleione is considerably smaller than the brightest stars in the Pleiades. Alcyone for instance has a radius that is 10 R_☉ with a luminosity 2,400 L_☉, making it roughly 30 times more voluminous than Pleione and about 13 times brighter.

Pleione is a classical Be star, often referred to as an "active hot star". Classical Be stars are B-type stars close to the main sequence with the "e" in the spectral type signifying that Pleione exhibits emission lines in its spectrum, rather than the absorption lines typical of B-type stars. Emission lines usually indicate that a star is surrounded by gas. In the case of a Be star, the gas is typically in the form of an equatorial disk, resulting in electromagnetic radiation that emanates not just from the photosphere, but from the disk as well. The geometry and kinematics of this gaseous circumstellar environment are best explained by a Keplerian disk – one that is supported against gravity by rotation, rather than gas or radiation pressure. Circumstellar disks like this are sometimes referred to as "decretion disks", because they consist of material being thrown off the star (as opposed to accretion disks which comprise material falling toward the star).