On Friday, Columbia Astronomy Public Outreach hosted a public lecture led by PhD candidate Jennifer Mead to share how astronomers unearth the secrets of dead stars, and how new ones emerge from their ashes. Enjoy a little astronomy trivia game at the end of our article!

On Friday, November 17, PhD candidate Jennifer Mead gave a public talk titled “Tales from the Stellar Graveyard: The Ghosts of Stars Past” at Pupin Hall as part of Columbia Astronomy Public Outreach’s first event of this  academic year. Due to the cloudy weather, Columbia Astronomy Public Outreach had to cancel its usual stargazing activity following the public talk. 

The event, open to the public, saw a great number of young kids with their parents joined in the auditorium, so Dr. Mead weaved a simile of life and death to animate the scientific topic of the collapse and rebirth of stars to make the science incredibly silly, fun, and easy to digest for the participants.

The event started with an engaging astronomy trivia. (Our staff editors have put them at the end of this post. Try them! The answers will be revealed in the comments section.)

Dr. Mead moved on to frame the death of stars as “a very spooky story about the ghosts of Stars past.” Dr. Mead portrayed a fairy-tale-like scene where “somewhere out in the vastness of space about 10 million years ago, a star and all of his little star buddies were born from a cloud of gas and dust. But in a story that is unfortunately tragic and very familiar to many stars, it ends with a bang.”

Dr. Mead introduced a number of ways stars can die. The first one was core collapse supernovae, for which Dr. Mead had a special shoutout to Star Trek fans. This type of death is a self-destruction that happens to massive stars with more than 8 times the mass of the sun. Stars undergo nuclear fusion in their cores, where atoms are fused to create larger atoms. However, this process  has limits, as it can only produce a certain number of elements before the star stops emitting energy. When the energy required to fuse atoms exceeds the energy produced, the outer layers of the star collapse inward, bounce off the core of the star, and explode outward.

Dr. Mead imagined this explosion to spread the “guts” of stars anywhere.

The second type of death for stars is White Dwarf Supernovae. Dr. Mead described it as  “cannibalism of another star”: a white dwarf would eat the outer layer of its “friend” when he gets a little bit too close. The dwarf ends up eating too much of its friend and explodes all of its guts all over the universe.

The third type is mergers, which Dr. Mead compared to the car crashes of the cosmos. “If you’re from New Jersey like I am, you know that circles are very dangerous,” she joked (for those who don’t know, New Jersey is notorious for having too many traffic circles and roundabouts).  If two stars circle against each other, their orbits will gradually spiral inward, getting closer until they crash and merge.

The last type was Planetary Nebulae. This happens to stars less than a few times the mass of the sun. Dr. Mead considered it an “extreme weight loss routine for stars”—at the end of these smaller stars’ lives, they start to eject the outer layers. The cores of these stars are the aforementioned white dwarf.

Stars’ “Weight Loss Routine”

Then, Dr. Mead prompted the audience to consider the size of a supernova in comparison to a nuclear bomb. A supernova is actually 10 to the 30th times greater than a nuclear explosion (which is 1 followed by 30 zeros, one trillion trillion million nuclear bombs). To give the audience an idea of what that number means, t it would take 5 billion earths, or 5000 suns, to stuff this many nuclear bombs

The size of these explosions is so immense that approximately 200 years after the bangs. The “star guts” typically extend to about 10 light-years—over twice the distance to our closest star. Dr. Mead described the scene as stars “[smearing] their guts all over the universe.” To further help the audience visualize, she showed a fascinating simulation of the first 400 million years after the Big Bang, made by her collaborators.

Dr. Mead likened the periodic table to a witch’s brew because the guts of stars spread everywhere, forming the dust and gas form a mixture of elements just like a soup. In fact, nearly every element in our universe was formed in stars.

Then Dr. Mead leveraged the astronomer’s periodic table to explain what elements were produced by each of the four different types of star “deaths.” The table identified elements born out of each corresponding type of “star death.” For astronomers, other than hydrogen, helium, and some of the human-made ones along the bottom, almost every line of elements came from the stars. 

Generated by our dying high-mass stars undergoing a self-destruct sequence, elements born from core collapse supernovae are known as Alpha elements. They earn this designation because they’re produced by colliding alpha particles with other atoms. An alpha particle is essentially the core or nucleus of a hydrogen atom, consisting of two protons, positively-charged particles, and two neutrons, which don’t carry an electric charge. By repeatedly adding these particles, a range of lighter elements can be created.

Moving on, the white dwarf supernovae, which involve the previously mentioned “cannibals,” yield elements known as the iron-peak elements (named for their proximity to iron on the periodic table by astronomers). Additionally, there are merging neutron stars. When two neutron-rich stars collide, numerous neutrons are released. These neutrons can then be attached to other elements. This attachment initiates beta decay, during which a neutron transforms into a proton and an electron, a negatively-charged particle. Changing the number of protons alters the element, and this rapid neutron capture process enables rapid movement up the periodic table by accumulating protons through beta decay.

Conversely, planetary nebulae yield a set of elements known as S-process elements. This slow neutron capture process is just the opposite of beta decay: it builds up elements on the periodic table at a slower pace due to the more measured rate of neutron capture, and so some lighter elements are produced as a result.

Further, Dr. Mead explained that every element emits a distinct set of colors, much like individual fingerprints. Astronomers use spectroscopy to study light by breaking it down into its different colors. The identified spectrum of colors would show variations in brightness at various wavelengths, including visible colors, as well as ultraviolet and infrared light. By examining these variations, astronomers can pinpoint the unique characteristics, or “fingerprints,” of different elements. 

Then Dr. Mead asked the audience to take up the diffraction gratings distributed to attendees before the event. These diffraction gratings are basic prisms that highlight the fingerprints of elements based on changes in brightness across different wavelengths. It’s essentially a piece of plastic that could break elements up into their component colors.

Light Tube Color Breakdown

Dr. Mead turned on a series of lamps, each of the light bulbs containing gas of a specific element. Then she asked the audience to hold the diffraction grating close to their eyes. When we saw the gas through the piece of plastic, we discovered that each unique gas element emanates a unique set of colors. from the scarlet and cyan bluish color of hydrogen, to the red-pink plus bright-yellow neon we are familiar with from witnessing the neon lights around New York, each view was distinctly different.

Breakdown of helium, mercury, nitrogen, and neon in the visible spectrum

When a star explodes, all of the colors in the visible spectrum are thrown together, creating a mysterious-looking, fluorescent watercolor ink scene.

Kepler’s Supernova Remnant, Puppis A Supernova Remnant, and Tycho’s Supernova Remnant

By understanding that each element has a distinct fingerprint, astronomers can assemble them like puzzle pieces to discern the lines observed in a spectrum and identify elements such as oxygen (purple), silicon (orange), magnesium (green), titanium (gray), etc.

Near the end, Dr. Mead gave us a glimmer of hope amid the discussion of stars’ death. Despite the emphasis on death, there is potential life after death. The elements from deceased stars, the remnants or “star guts” dispersed into the universe, would blend into clouds of gas and dust. As these clouds cool down, they give rise to new stars. 

This is the origin of our sun and our own existence, and this cycle of birth and death will be the destiny of our Earth.

Pillars of Creation

Dr. Mead circled the new stars forming (the white little spots here and there) in this picture from NASA’s James Webb Space Telescope’s mid-infrared view. The infrared imager allows astronomers to see through the dust and gas clouds in this wavelength and discern the stars emerging at the tips of the pillars and blowing away surrounding gas and dust with their stellar wind. It signifies the creation of new stars from the remnants of older ones.

Dr. Mead concluded her speech with a playful, clever twist of famous American astronomer Carl Sagan’s iconic statement “We are made of star stuff”; for her and her ghost of stars, “We are made of star guts.”

Images via Bwog Staff, Chandra X-ray Observatory, Webb Telescope, NASA SVS, Jet Propulsion Lab

Trivia Qs

1. Most luminous matter in the universe is composed of hydrogen and helium. All elements heavier than helium are referred to as “metals” by astronomers. How much of the universe’s luminous mass is composed of metals?

A) 0.02%

B) 0.2%

C) 2%

D) 20%

2. The majority of the elements on the periodic table were originally produced by stars, either during their lives or during their deaths. Which stellar process is the main producer of gold in our universe?

A) The explosion of a white dwarf (a very tiny star)

B) The explosion of a very massive star

C) The collision of two neutron stars (very tiny stars)

D) The merger of two black holes

3. At the end of the lifetime of a very massive star, the star’s core will collapse in on itself, producing a brilliant”core-collapse” supernova explosion. Roughly how long does this process of core collapse take?

A) 1 second

B) 1day

C) 10 years

D) 10,000 years

4. When a white dwarf star becomes too massive (usually by stealing mass from a nearby companion star), it explodes as a“Type la” supernova. The brightest observed stellar event in recorded history – SN 1006 – was the result of a Type la supernova. Roughly how much brighter was SN 1006 than Venus?

A) 2 times

B) 20 times

C) 200 times

D) 2000 times

5. Which of the following most closely resembles the futurestate of the Sun, immediately following its “death”?

A) B) C) D)

6. A supernova explosion occurs inside the Milky Way roughly once every century. The Crab Nebula (shown in the background) is the remnant of one of these supernovae. Chinese astronomers observed this explosion in 1054 AD -but how long ago did the explosion actually occur?

A) 969 years ago

B) 7,500 years ago

C) 105000 years ago

D) 2.6 million years