T Coronae Borealis is a well-known recurrent nova. a. Is it a single star or a binary system? Explain. b. What mechanism causes a nova to flare up? c. How can a nova flare-up happen more than once?

T Coronae Borealis is a fascinating example of a recurrent nova, and it is part of a binary star system. But what exactly is a binary star system?

This term refers to two stars that are gravitationally bound to each other and orbit around a common center of mass.
In these systems, one of the stars is usually a white dwarf, while the other can be a main-sequence star, a red giant, or even another white dwarf.

In the case of T Coronae Borealis, there's a white dwarf and a giant star. The interaction between these two stars is key to understanding why T Coronae Borealis can flare up repeatedly.

• The primary star in our scenario is the white dwarf.
• The secondary star is usually larger and supplies material to the white dwarf.

These two stars, orbiting closely, provide the essential ingredients for a recurrent nova.

A nova flare-up is a dramatic and sudden increase in brightness, but what causes this to happen? It all boils down to a mechanism called thermonuclear runaway.
For thermonuclear runaway to occur, material from the secondary star is pulled onto the white dwarf.

This material primarily comprises hydrogen, which accumulates on the white dwarf's surface.
As more and more hydrogen piles up, the pressure and temperature at the bottom of this layer increase.
Eventually, the conditions become extreme enough for hydrogen nuclei to undergo nuclear fusion.

When this fusion starts, it happens in a runaway manner, meaning the reaction rapidly escalates and releases a tremendous amount of energy.
This sudden release of energy causes the nova to flare up, dramatically increasing its brightness observable from Earth.

• First, hydrogen accumulates on the white dwarf.
• Next, pressure and temperature build up.
• Finally, nuclear fusion kicks in, releasing colossal energy.

This sequence is what we call a thermonuclear runaway.

One of the key reasons T Coronae Borealis can flare up multiple times lies in the process called mass transfer.
So, what is mass transfer, and how does it allow for recurrent flare-ups? Mass transfer occurs when the gravity of the white dwarf pulls material from its companion star.

Imagine the companion star overflowing its 'Roche lobe', the region around it where material is gravitationally bound to the star. When material spills over this boundary, the white dwarf's intense gravity captures it.

This captured material, rich in hydrogen, then collects on the white dwarf's surface.
Over time, as described earlier, this hydrogen undergoes thermonuclear runaway, leading to a nova flare-up.

When the process of mass transfer and hydrogen accumulation happens more than once, you get recurrent nova events.

• Mass transfer involves material moving from the secondary star to the white dwarf.
• This material accumulates over time.
• Repeated cycles of mass transfer allow multiple nova events.

This cycle can take years or even decades, making recurrent novae like T Coronae Borealis extremely captivating objects to study in astronomy.