Two new views highlight the dual nature of star AG Carinae

Comparing two images shows blown dust bubbles and an erupting gas shell, the last acts of a monster star’s life.

These new views showcase the dual nature of the star AG Carinae, which was the target of the NASA / ESA Hubble Space Telescope’s 31st anniversary image in April 2021. This new perspective is the result of the star’s observations by Hubble in 2020 and 2014, as well as with others captured by the telescope’s wide-field planetary camera 2 in 1994.

The first image shows the details of the ionized hydrogen and nitrogen emissions from the expanding nebula shell (seen here in red). In the second image, the color blue defines the distribution of dust that shines in the reflected light from the stars. Astronomers believe that powerful stellar winds from the star formed and shaped the dust bubbles and filaments. The nebula is about five light years wide, similar to the distance from here to the nearest star beyond the Sun, Proxima Centauri.

AG Carinae is formally classified as a bright blue variable because it is a hot bright (emitting blue light) star with varying brightness. Such stars are quite rare because few are so massive. Bright blue variable stars continually lose mass in the later stages of life. The star is waging a fierce struggle between gravity and the pressure of radiation to avoid self-destruction. When the star begins to run out of fuel, its radiation pressure decreases and gravity begins to set in. Stellar matter succumbs to gravity and falls inward. It heats up and is explosively ejected into the surrounding interstellar space. This process continues until sufficient mass is lost and the star reaches a stable state.

The spectacular nebula surrounding AG Carinae formed by matter ejected from the star during several of its past explosions. The nebula is about 10,000 years old, and the observed gas speed is about 43 miles per second. Although this nebula looks like a ring, it is actually a hollow shell whose center has been cleaned of gas and dust by a powerful stellar wind moving at around 124 miles per second. The gas (composed mostly of ionized hydrogen and nitrogen) in these images appears as a thick, bright red ring, which appears doubled in places, likely the result of multiple explosions colliding. The dust, seen here in blue, formed into tufts, bubbles, and filaments and was shaped by the stellar wind.

Scientists who have observed the star and its surrounding nebula find that the ring is not perfectly spherical. It appears to have bipolar symmetry. This could mean that the mechanism producing the explosion may be the result of a disc in the center, or that the star may have a companion (known as a binary star). An alternative and simpler theory is that, like many massive stars, AG Carinae can spin very quickly.