Listening Section 4: Note & Table Completion: Question 8

Transcript-based practice (no audio).

Lecturer: Today's lecture is about brown dwarfs, sometimes nicknamed "failed stars," because they sit in an awkward middle ground between the largest planets and the smallest true stars.

To understand why, we need to talk about mass. A brown dwarf typically has a mass somewhere between about thirteen and eighty times the mass of Jupiter. Below that lower boundary, an object is generally classified as a planet; above the upper boundary, it becomes massive enough to fuse ordinary hydrogen in its core and shine as a genuine star.

Brown dwarfs do undergo one type of nuclear fusion, though: they can briefly fuse deuterium, a heavier isotope of hydrogen, early in their lives. This produces a faint glow, but nowhere near the sustained brightness of a main-sequence star, which is why brown dwarfs are notoriously difficult to detect.

The first confirmed brown dwarf, named Teide 1, wasn't found until 1995, more than three decades after the category was first proposed theoretically. It was discovered in the Pleiades star cluster using ground-based telescopes sensitive to infrared light, since brown dwarfs radiate most of their energy at infrared wavelengths rather than visible ones.

Since then, astronomers have organised brown dwarfs into three spectral classes, based on decreasing temperature: L, T, and Y, with Y-class objects being the coolest and dimmest, some barely warmer than a household oven.

One landmark discovery came in 2013, when astronomers identified a pair of brown dwarfs, now called Luhman 16, orbiting each other just six point five light years from Earth, making it the third-closest star system to our own Sun.

What's particularly exciting to atmospheric scientists is that brown dwarfs appear to have genuine weather. Observations of changing brightness over time suggest cloud bands of hot silicate or iron droplets that shift and break apart, not unlike storm systems on Jupiter, though far more extreme.

Large-scale surveys have been essential to this field. The Two Micron All Sky Survey, usually abbreviated to 2MASS, catalogued thousands of candidate objects in the late 1990s and remains a key reference dataset for brown dwarf research today.

Looking forward, the James Webb Space Telescope's infrared instruments are expected to characterise brown dwarf atmospheres in far greater detail over the coming decade, potentially settling the ongoing debate about where exactly the line between a large planet and a small brown dwarf should be drawn.

Complete the notes below. Write NO MORE THAN TWO WORDS AND/OR A NUMBER for each answer.

BROWN DWARFS: LECTURE NOTES

Definition

• Minimum mass to count as a brown dwarf: about 1 __________ Jupiter masses
• Above the top of this range, an object can fuse ordinary 2 __________ and becomes a true star

Fusion

• Brown dwarfs can briefly fuse 3 __________ early in their lives

Discovery

• First confirmed brown dwarf, Teide 1, found in 4 __________ (year)
• Found using telescopes sensitive to 5 __________ light

Nearby system

• Luhman 16 lies about 6 __________ light years from Earth

Key survey

• Large-scale survey used: 7 __________