What Is the Strongest Magnet in the Universe?
The short answer
The strongest magnet we know of is a magnetar: a dead star about 20 km (12.5 miles) across, the size of a city. Its magnetic field is about a thousand trillion times Earth's. That is roughly a billion times the 16 tesla magnet that floated a live frog in 1997.
Up close, that field is deadly. Within about 1,000 km, it would squeeze the atoms in your body out of shape. Within about 100 km, they would be squeezed into thin needles.
From a floating frog to a dead star
In 1997, scientists in the Netherlands floated a live young frog inside a 16 tesla magnet. Frogs and people are mostly water, and a strong magnetic field pushes water away. The frog was unharmed.
Our strongest steady magnet reached 48.7 tesla in 2025. The record came from a small superconducting test coil that added 17.6 tesla inside a 31 tesla magnet. That is about three times the frog's magnet.
Magnetar fields are estimated at 10 billion to 100 billion tesla. Divide by 16 and you get about 625 million to 6.25 billion times the frog's magnet. The film rounds that to a billion.
Why your card would lose its stripe
Credit cards often use high coercivity stripes. It takes a field of about 2,750 oersteds to wipe one. A magnetar's field fades fast with distance, but it starts out so strong that a typical one on the far side of Earth would still be 9 to 175 times over that line. The weakest known magnetars only just reach it.
Only the stripe is lost. Chip and tap payments still work. In 2021, Mastercard said it would issue no new cards with stripes from 2029.
The starquake that reached Earth
On December 27, 2004, a magnetar about 28,000 light years away, near the middle of our galaxy, let out a giant flare. Its twisted field cracked its crust: a starquake. For 0.2 seconds, it outshone the full Moon.
NASA's 2005 report said it released more energy in a tenth of a second than the Sun gives off in 150,000 years. That figure assumed a distance of 50,000 light years. At today's closer estimate, it comes to tens of thousands of years.
The flash overloaded, or dazzled, satellite detectors. It lit up Earth's upper atmosphere and bounced off the Moon. One satellite caught the echo from the Moon 2.8 seconds later.
Key numbers
| What | Number | Source |
|---|---|---|
| Magnetar field compared with Earth's | about a thousand trillion times | NASA HEASARC |
| Size of a neutron star such as a magnetar | about 20 km (12.5 miles) across | NASA Imagine the Universe |
| Field that floated the live frog, 1997 | 16 tesla | Berry and Geim, 1997 |
| Strongest steady magnet, 2025 | 48.7 tesla (a 17.6 tesla test coil inside a 31 tesla magnet) | National High Magnetic Field Laboratory, 2025 |
| Magnetar field compared with the frog's magnet | about 625 million to 6.25 billion times | Our arithmetic from Duncan, University of Texas at Austin, and Berry and Geim, 1997 |
| Distance where the field squeezes atoms out of shape | within about 1,000 km | Duncan, University of Texas at Austin |
| Distance where atoms become thin needles | within about 100 km | Our arithmetic from Duncan, University of Texas at Austin, and Lai, 2001 |
| Field needed to wipe a high coercivity card stripe | about 2,750 oersteds | CardLogix |
| Field of a typical magnetar on the far side of Earth, compared with what wipes a stripe | 9 to 175 times | Our arithmetic from Duncan, University of Texas at Austin, and CardLogix |
| Distance to the magnetar of the 2004 flare | about 8.7 kiloparsecs (about 28,000 light years) | McGill Online Magnetar Catalog |
| Energy of the 2004 flare, NASA's 2005 figure at an assumed 50,000 light years | more than the Sun gives off in 150,000 years, released in a tenth of a second | NASA Swift, 2005 |
| Delay of the flare's echo off the Moon | 2.8 seconds | Mereghetti et al., 2005 |
What the film simplifies
- The film says the strongest magnet we know of, not the strongest there is. A magnetar has the most intense field yet observed, which doesn't prove nothing stronger exists.
- A billion times is a round number. Depending on the magnetar, it is about 625 million to 6.25 billion times the frog's magnet.
- A magnetar on the far side of Earth is a thought experiment. Its gravity would wreck Earth long before your card mattered. The line is only about its field, and the field wipes only the stripe, not the chip.
- Our strongest steady magnet is a small test coil working inside a bigger 31 tesla magnet. Together they reach 48.7 tesla.
- The film says the flash shook Earth's upper atmosphere. NASA's report says it lit it up. The flash hit Earth directly, and the Moon only sent back an echo, 2.8 seconds later.
- The frog is a cartoon of the real 1997 frog, and no frog could feel the flare. The magnetar's hair is its field lines, Earth's field is drawn as a comb over, and the needles inside the fingertip are drawn far bigger than atoms.
At a glance
Runtime 1 minute 15 seconds · Topics: Magnetism, Magnetars, Neutron stars · Captions on YouTube · Full script below
Test yourself
Why can a strong magnet lift a frog at all?
Answer
Frogs, like people, are mostly water, and a strong magnetic field pushes water away. In a 16 tesla magnet that push was enough to float a live frog.
The strongest steady magnet reached 48.7 tesla in 2025. About how many times stronger is that than the frog's 16 tesla magnet?
Answer
About 3 times, because 48.7 divided by 16 is about 3.0.
A magnetar on the far side of Earth would wipe the stripe on your credit card. Could you still pay with that card?
Answer
Yes, by chip or tap. Even that far away, a typical magnetar's field is 9 to 175 times the field that wipes a high coercivity stripe, so the stripe is lost, but the chip and tap still work.
The script
Host: A magnet floated this frog. For real.
Frog: Best float of my life. Now I want the strongest magnet in the universe.
Host: He's mostly water. So are you. Strong magnets push water away.
Frog: So I didn't fly. A magnet rejected me. Upward.
Host: Our strongest steady magnet? Three times the one that floated him.
Frog: Weak. Make it a billion.
Host: A billion? That's a magnetar.
Host: The strongest magnet we know of. A dead star about the size of a city.
Frog: Look at that hair. Float me.
Host: Within about a thousand kilometers, its field would squeeze your atoms out of shape.
Host: Closer, and they're needles.
Frog: Needles? Fine. I can live with needles.
Host: Put one on the far side of Earth.
Host: It would still wipe the stripe on the credit card in your pocket.
Frog: Not the card. That card has points.
Host: They're all thousands of light years away. One still reached us.
Host: In 2004, one got its field so twisted, its crust cracked.
Host: A starquake. Its flash dazzled the satellites built to watch for it.
Frog: Do it again.
Host: It shook Earth's upper atmosphere and bounced off the Moon.
Frog: I felt that.
Host: You didn't.
Frog: Best float of my life.
Sources
- A live young frog floated unharmed inside a 16 tesla magnet in 1997.Berry and Geim, 1997
- The steady field record: 48.7 tesla, a superconducting test coil adding 17.6 tesla inside a 31 tesla magnet.National High Magnetic Field Laboratory, 2025
- Magnetar fields are about a thousand trillion times Earth's; the most intense magnetic field yet observed.NASA HEASARC
- The biggest stars become the strongest magnets when they die: magnetars.Harvard Smithsonian Center for Astrophysics, 2005
- Earth's magnetic field at the surface is 22,000 to 67,000 nT.British Geological Survey
- Neutron stars are about 20 km (12.5 miles) across, the size of a city.NASA Imagine the Universe
- Magnetar fields of 100 trillion to 1,000 trillion gauss (10 billion to 100 billion tesla); such fields strongly distort atoms, a magnetar within 1,000 km would be lethal, and stronger fields squeeze atoms into thin needles.Duncan, University of Texas at Austin
- How atoms and matter behave in very strong magnetic fields.Lai, 2001
- Adults are up to 60% water.USGS Water Science School
- High coercivity stripes are encoded with a field of about 2,750 oersteds; credit cards often use them.CardLogix
- Mastercard is phasing out the magnetic stripe, with no new striped credit or debit cards from 2029.GovInfoSecurity, 2021
- The giant flare of December 27, 2004 bounced off the Moon and lit up Earth's upper atmosphere; more energy in a tenth of a second than the Sun gives off in 150,000 years at 50,000 light years.NASA Swift, 2005
- The flare came from a failure of the star's crust: a starquake.Hurley et al., 2005
- The flare outshone the full Moon for 0.2 seconds.Palmer et al., 2005
- The flare saturated the satellite's detector, and its echo off the Moon was detected 2.8 seconds later.Mereghetti et al., 2005
- The 2004 magnetar's distance is about 8.7 kiloparsecs.McGill Online Magnetar Catalog