The Rundown — September 19, 2026

What's going on right now, in one place. Items stay plain text until one of them becomes its own entry below — then it turns into a link.

Warren Buffett, 96, steps down as chairman of Berkshire Hathaway after more than 60 years leading the company, taking the title of chairman emeritus; his son Howard G. Buffett, a Berkshire director for 33 years, becomes chairman, while Greg Abel continues as CEO, a role he took over from Buffett eight months ago — I took a closer look at the S&P 500 advice Buffett leaves behind. Read the entry
SpaceX's weighting in the Nasdaq-100 is set to more than double, from about 1.28% to roughly 2.82%, when the index's rebalance takes effect at Monday's close, September 21, after lockup expirations pushed the company's public float to about 16% of shares outstanding.

Bitcoin trades above $81,000 on September 19, extending a rally that shrugged off both this week's Fed rate hike and the Senate's failure to invoke cloture on the CLARITY Act digital-asset market-structure bill, which fell short of the 60 votes needed to advance.
The rally coincided with reports that the Commodity Futures Trading Commission sent its own crypto market-structure rules to the White House for review.

Anthropic makes Claude Sonnet 5's introductory API pricing — $2 per million input tokens, $10 per million output tokens — permanent, canceling a previously announced increase to $3/$15 that had been set to take effect September 1.
Apple's new Mac Studio, with M5 Max and M5 Ultra chip options, begins shipping September 22 starting at $2,499 — in the exact same case Apple's used since 2023, it turns out; I looked at what actually changed. Read the entry

A Nature Communications modeling study finds that continued political fragmentation of Latin America's power grids raises system costs by up to 7% and weakens drought resilience, while regional grid integration could save $32 billion a year and sharply cut emissions by 2045.
Using the James Webb Space Telescope, astronomers searching the star-forming region IC 348 find brown dwarfs with just twice Jupiter's mass, pushing the study of these objects into a new, lower mass range — and possibly below the mass floor theory says a failed star should be able to form at, which I dug into. Read the entry

President Trump signs the Lindsey O. Graham Sanctioning Russia and Iran Act into law, authorizing tariffs of up to 100% on the five largest buyers of Russian oil and gas and sanctioning Vladimir Putin, Russian officials, oligarchs and banks; the bill is named for the senator who negotiated it before his death in July.
Russian guided bombs hit an apartment building in Sumy, killing two and injuring seven, including a 13-year-old girl, the same day a separate strike near Nikopol injures seven more.

The FDA approves levacetylleucine (Aqneursa) as the first treatment for ataxia in patients with ataxia-telangiectasia, based on phase 3 trial data showing a statistically significant improvement (P<.001) on a standard ataxia rating scale within 12 weeks — I wrote up what the disease actually is and what the trial showed. Read the entry

First published on the Notebook, September 19, 2026.

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